QUAD FM4 - bought in Europe, can I use it un US?

Correct, nothing else needs changing.

Thanks for the responses.

So, there is no recalibration that needs to take place for the frequency bands, etc, on the analog side of the receiver, ie, the tuning stage?

I moved from the UK to NA 25 years ago, with my QUAD FM4/44/405 system. Yes, the FM4 will work fine here. However, the FM de-emphasis in NA is different to that in Europe, 75µS in NA versus 50µS for most of the world. Consequently a European tuner will sound slightly bass heavy if used in NA. The fix is simple, a single capacitor addition in the de-emphasis circuit. I had mine done by QUAD before I left the UK. I am sure QS&D in the States can do the job for you at nominal expense. Re voltage conversions, my FM4 is dual voltage, so no problem here. If yours is not, use a $25 step-up transformer or have QS&D do the conversion for you. I am not familiar with the newer FM4 tuners, maybe it is a simple jumper change you can do yourself to go from 240V to 110V

I don't think you can. However you'll have to wait for more expert advice. I'm not into tuners. All I know is, that it will need more modification than just changing transformers.