Since the RPi 4 has a micro hdmi output you would have to be using an adapter of some form to connect with the same cable you use for your RPi 3. It sounds to me like the issue is most likely this adapter not passing pin 13 across which isn’t really unheard of and would keep CEC from working. As long as you are sure CEC is enabled (settings>system>input>peripherals) I would recommend acquiring a proper micro hdmi to full size hdmi cable.