To clarify, Raspberry Pi 4B is supported for both MATLAB & Simulink from R2020a.
Coming to the SSH issue, I hope you have done the Hardware Setup process and loaded the MathWorks image to SD Card. In the setup process, after you insert the SD Card back in the Pi, power it up and click the next button, MATLAB tries to do the SSH and other needed checks for the newly booted pi.
It is possible that the raspberry pi 4b with buster image could take a little longer(3-4min) to boot for the first time. Although ping works, SSH could fail till it boots up fully. If possible, I would suggest, connect a monitor once during boot time to see if things are normal.