This is the:

DasMikro DSK-309 Futaba T-FHSS Nano 4CH Antenna-Free Receiver

(1.5mm JST)
21mm x 12mm

Compatible Transmitters:

FUTABA 7PX / FUTABA 7PXR
FUTABA 4PX / FUTABA 4PXR
FUTABA 4PV / FUTABA 3PV
FUTABA 4PLS / FUTABA 4PL
FUTABA 4PM
FUTABA 7XC / FUTABA 4GRS