diff --git a/examples/FSK4/FSK4_Transmit/FSK4_Transmit.ino b/examples/FSK4/FSK4_Transmit/FSK4_Transmit.ino
new file mode 100644
index 00000000..045469cd
--- /dev/null
+++ b/examples/FSK4/FSK4_Transmit/FSK4_Transmit.ino
@@ -0,0 +1,122 @@
+/*
+   RadioLib FSK4 Transmit Example
+
+   This example sends an example FSK-4 'Horus Binary' message using SX1278's
+   FSK modem.
+
+   This signal can be demodulated using a SSB demodulator (SDR or otherwise), and
+   horusdemodlib: https://github.com/projecthorus/horusdemodlib/wiki
+
+   Other modules that can be used for FSK4:
+   (Untested, but work with RTTY to are likely to work here too)
+    - SX127x/RFM9x
+    - RF69
+    - SX1231
+    - CC1101
+    - SX126x
+    - nRF24
+    - Si443x/RFM2x
+    - SX128x
+
+   For default module settings, see the wiki page
+   https://github.com/jgromes/RadioLib/wiki/Default-configuration
+
+   For full API reference, see the GitHub Pages
+   https://jgromes.github.io/RadioLib/
+*/
+
+// include the library
+#include <RadioLib.h>
+
+// SX1278 has the following connections:
+// NSS pin:   10
+// DIO0 pin:  2
+// RESET pin: 9
+// DIO1 pin:  3
+SX1278 radio = new Module(10, 2, 9, 3);
+
+// or using RadioShield
+// https://github.com/jgromes/RadioShield
+//SX1278 radio = RadioShield.ModuleA;
+
+// create FAK4 client instance using the FSK module
+FSK4Client fsk4(&radio);
+
+// A 'canned' encoded Horus Binary telemetry packet.
+// Refer here for packet format information: 
+// https://github.com/projecthorus/horusdemodlib/wiki/2---Modem-Details#horus-binary-v1-mode-4-fsk
+// After demodulation, deinterleaving, and descrambling, this results in a packet: 00000001172D0000000000000000D20463010AFF2780
+// This decodes to the Habitat-compatible telemetry string: $$4FSKTEST,0,01:23:45,0.00000,0.00000,1234,99,1,10,5.00*ABCD
+uint8_t sample_packet[] = {
+    0x45, 0x24, 0x24, 0x48, 0x2F, 0x12, 0x16, 0x08, 0x15, 0xC1,
+    0x49, 0xB2, 0x06, 0xFC, 0x92, 0xEB, 0x93, 0xD7, 0xEE, 0x5D,
+    0x35, 0xA0, 0x91, 0xDA, 0x8D, 0x5F, 0x85, 0x6B, 0x63, 0x03,
+    0x6B, 0x60, 0xEA, 0xFE, 0x55, 0x9D, 0xF1, 0xAB, 0xE5, 0x5E,
+    0xDB, 0x7C, 0xDB, 0x21, 0x5A, 0x19
+};
+uint8_t sample_packet_len = 45;
+
+void setup() {
+  Serial.begin(9600);
+
+  // initialize SX1278 with default settings
+  Serial.print(F("[SX1278] Initializing ... "));
+  int state = radio.beginFSK();
+
+  // when using one of the non-LoRa modules for FSK4
+  // (RF69, CC1101, Si4432 etc.), use the basic begin() method
+  // int state = radio.begin();
+
+  if(state == ERR_NONE) {
+    Serial.println(F("success!"));
+  } else {
+    Serial.print(F("failed, code "));
+    Serial.println(state);
+    while(true);
+  }
+
+  // initialize FSK4 client
+  // NOTE: FSK4 frequency shift will be rounded
+  //       to the nearest multiple of frequency step size.
+  //       The exact value depends on the module:
+  //         SX127x/RFM9x - 61 Hz
+  //         RF69 - 61 Hz
+  //         CC1101 - 397 Hz
+  //         SX126x - 1 Hz
+  //         nRF24 - 1000000 Hz
+  //         Si443x/RFM2x - 156 Hz
+  //         SX128x - 198 Hz
+  Serial.print(F("[FSK4] Initializing ... "));
+  // low ("space") frequency:     434.0 MHz
+  // frequency shift:             270 Hz (actually results in a shift of 244 Hz)
+  // baud rate:                   100 baud
+  state = fsk4.begin(434.0, 270, 100);
+  if(state == ERR_NONE) {
+    Serial.println(F("success!"));
+  } else {
+    Serial.print(F("failed, code "));
+    Serial.println(state);
+    while(true);
+  }
+
+}
+
+void loop() {
+  Serial.print(F("[FSK4] Sending FSK4 data packet ... "));
+
+  // send out idle condition for 1000 ms
+  fsk4.idle();
+  delay(1000);
+
+  // FSK4Client supports the write(uint8_t b) and write(uint8_t* buff, size_t len) method.
+  // We use the write(uint8_t b) method to send a few bytes of preamble to allow the demodulator
+  // to lock on to the signal.
+  fsk4.write(0x1B); fsk4.write(0x1B); fsk4.write(0x1B); fsk4.write(0x1B);
+  fsk4.write(0x1B); fsk4.write(0x1B); fsk4.write(0x1B); fsk4.write(0x1B);
+
+  // We then send the encoded packet.
+  fsk4.write(sample_packet, sample_packet_len);
+
+  Serial.println(F("done!"));
+
+}
diff --git a/src/RadioLib.h b/src/RadioLib.h
index e8e26ad0..558eec22 100644
--- a/src/RadioLib.h
+++ b/src/RadioLib.h
@@ -98,6 +98,7 @@
 #include "protocols/Morse/Morse.h"
 #include "protocols/RTTY/RTTY.h"
 #include "protocols/SSTV/SSTV.h"
+#include "protocols/FSK4/FSK4.h"
 
 // transport layer protocols
 #include "protocols/TransportLayer/TransportLayer.h"
diff --git a/src/protocols/FSK4/FSK4.cpp b/src/protocols/FSK4/FSK4.cpp
new file mode 100644
index 00000000..df331d9c
--- /dev/null
+++ b/src/protocols/FSK4/FSK4.cpp
@@ -0,0 +1,119 @@
+#include "FSK4.h"
+#if !defined(RADIOLIB_EXCLUDE_FSK4)
+
+
+
+FSK4Client::FSK4Client(PhysicalLayer* phy) {
+  _phy = phy;
+  #if !defined(RADIOLIB_EXCLUDE_AFSK)
+  _audio = nullptr;
+  #endif
+}
+
+//#if !defined(RADIOLIB_EXCLUDE_AFSK)
+// FSK4Client::FSK4Client(AFSKClient* audio) {
+//   _phy = audio->_phy;
+//   _audio = audio;
+// }
+//#endif
+
+int16_t FSK4Client::begin(float base, uint32_t shift, uint16_t rate) {
+  // save configuration
+  _baseHz = base;
+  _shiftHz = shift;
+
+
+  // calculate duration of 1 bit
+  _bitDuration = (uint32_t)1000000/rate;
+
+  // calculate module carrier frequency resolution
+  uint32_t step = round(_phy->getFreqStep());
+
+  // check minimum shift value
+  if(shift < step / 2) {
+    return(ERR_INVALID_RTTY_SHIFT);
+  }
+
+  // round shift to multiples of frequency step size
+  if(shift % step < (step / 2)) {
+    _shift = shift / step;
+  } else {
+    _shift = (shift / step) + 1;
+  }
+
+  // Write resultant tones into arrays for quick lookup when modulating.
+  _tones[0] = 0;
+  _tones[1] = _shift;
+  _tones[2] = _shift*2;
+  _tones[3] = _shift*3;
+
+  _tonesHz[0] = 0;
+  _tonesHz[1] = _shiftHz;
+  _tonesHz[2] = _shiftHz*2;
+  _tonesHz[3] = _shiftHz*3;
+
+  // calculate 24-bit frequency
+  _base = (base * 1000000.0) / _phy->getFreqStep();
+
+  // configure for direct mode
+  return(_phy->startDirect());
+}
+
+void FSK4Client::idle() {
+  // Idle at Tone 0.
+  tone(0);
+}
+
+size_t FSK4Client::write(uint8_t* buff, size_t len) {
+  size_t n = 0;
+  for(size_t i = 0; i < len; i++) {
+    n += FSK4Client::write(buff[i]);
+  }
+  FSK4Client::standby();
+  return(n);
+}
+
+size_t FSK4Client::write(uint8_t b) {
+
+    int k;
+    // Send symbols MSB first.
+    for (k=0;k<4;k++)
+    {
+        // Extract 4FSK symbol (2 bits)
+        uint8_t symbol = (b & 0xC0) >> 6;
+        // Modulate
+        FSK4Client::tone(symbol);
+        // Shift to next symbol.
+        b = b << 2;
+    }
+
+  return(1);
+}
+
+void FSK4Client::tone(uint8_t i) {
+  uint32_t start = Module::micros();
+  transmitDirect(_base + _tones[i], _baseHz + _tonesHz[i]);
+  while(Module::micros() - start < _bitDuration) {
+    Module::yield();
+  }
+}
+
+int16_t FSK4Client::transmitDirect(uint32_t freq, uint32_t freqHz) {
+  #if !defined(RADIOLIB_EXCLUDE_AFSK)
+  if(_audio != nullptr) {
+    return(_audio->tone(freqHz));
+  }
+  #endif
+  return(_phy->transmitDirect(freq));
+}
+
+int16_t FSK4Client::standby() {
+  #if !defined(RADIOLIB_EXCLUDE_AFSK)
+  if(_audio != nullptr) {
+    return(_audio->noTone());
+  }
+  #endif
+  return(_phy->standby());
+}
+
+#endif
diff --git a/src/protocols/FSK4/FSK4.h b/src/protocols/FSK4/FSK4.h
new file mode 100644
index 00000000..eb83c66e
--- /dev/null
+++ b/src/protocols/FSK4/FSK4.h
@@ -0,0 +1,82 @@
+#if !defined(_RADIOLIB_FSK4_H)
+#define _RADIOLIB_FSK4_H
+
+#include "../../TypeDef.h"
+
+#if !defined(RADIOLIB_EXCLUDE_FSK4)
+
+#include "../PhysicalLayer/PhysicalLayer.h"
+#include "../AFSK/AFSK.h"
+
+
+/*!
+  \class FSK4Client
+
+  \brief Client for FSK-4 communication. The public interface is the same as Arduino Serial.
+*/
+class FSK4Client {
+  public:
+    /*!
+      \brief Constructor for FSK-4 mode.
+
+      \param phy Pointer to the wireless module providing PhysicalLayer communication.
+    */
+    explicit FSK4Client(PhysicalLayer* phy);
+
+    #if !defined(RADIOLIB_EXCLUDE_AFSK)
+    /*!
+      \brief Constructor for AFSK mode.
+
+      \param audio Pointer to the AFSK instance providing audio.
+    */
+    //explicit FSK4Client(AFSKClient* audio);
+    #endif
+
+    // basic methods
+
+    /*!
+      \brief Initialization method.
+
+      \param base Base (space) frequency to be used in MHz (in FSK-4 mode), or the space tone frequency in Hz (in AFSK mode)
+
+      \param shift Frequency shift between each tone in Hz.
+
+      \param rate Baud rate to be used during transmission.
+
+
+      \returns \ref status_codes
+    */
+    int16_t begin(float base, uint32_t shift, uint16_t rate);
+
+    /*!
+      \brief Send out idle condition (RF tone at mark frequency).
+    */
+    void idle();
+
+    size_t write(uint8_t* buff, size_t len);
+    size_t write(uint8_t b);
+
+
+#ifndef RADIOLIB_GODMODE
+  private:
+#endif
+    PhysicalLayer* _phy;
+    #if !defined(RADIOLIB_EXCLUDE_AFSK)
+    AFSKClient* _audio;
+    #endif
+
+    uint32_t _base = 0, _baseHz = 0;
+    uint32_t _shift = 0, _shiftHz = 0;
+    uint32_t _bitDuration = 0;
+    uint32_t _tones[4];
+    uint32_t _tonesHz[4];
+
+    void tone(uint8_t i);
+
+    int16_t transmitDirect(uint32_t freq = 0, uint32_t freqHz = 0);
+    int16_t standby();
+};
+
+#endif
+
+#endif