Do you hear yourself using the local echo feature of the OS4 also using the FTM400? If not, then please run C4FM AutoCal, or manually adjust the RX offset on the Modem page in +-100Hz steps from -500Hz to +500Hz for the lowest BER. You can verify settings using the openSPOT's built-in local echo service.
If AutoCal does not work for you then the RX offset between the transceiver and the openSPOT is greater than 500Hz. What you can do is to switch frequencies on the openSPOT, but keep the transceiver's frequency. For example, if the transceiver is on 433.900MHz, then test by setting the openSPOT to the following frequencies:
433900500 Hz
433900600 Hz
433900700 Hz
...
433899500 Hz
433899400 Hz
433899300 Hz
...