CDG lounge: No access. It is not a BA lounge. BA have been allocated very little space in the lounge that they're sharing, and have restricted access to this. Non-BA OW Sapphires get no access, unfortunately. BA are embarrassed by the situation but there's nothing they can do at the moment.

LHR lounge: The T1 lounge is large and very nice. There was a big revamp when BA moved some long-haul flights into T1, and everyone using T1 gets the benefit of this.

Connecting: You do not have to go out of the terminal, but you will have to clear security. On arriving at T4, follow the signs for Flight Connections T1. This will put you onto a bus which takes you to the Flight Connections Centre at T1. Go up to the top. Clear security. If you don't, for some reason, have a LHR-FCO boarding pass, you can collect one at the desks immediately beyond security - but if you have lounge access, don't bother. Follow the signs down one level and into T1 departures. As soon as you exit the seating area into the shopping area of T1, turn hard left to get to the lounge entrance.

Seat pre-allocation: Quick answer: Not in economy on short-haul international routes.