Overnight stay in Terminal 4 at London Heathrow?

Yes you will need to go through immigration and customs terminal four is a train/bus ride away from terminals 1,2,3 and 2 train rides from terminal 5. If you have your tickets and boarding p you don't need to check in - just head for security in departures. If you don't have a boarding p you will need to check in from 2 hours before your flight. The facilities at terminal 4 are not great you will find more 24 hour facilities at terminals 1,3 and 5