Probably a 777. It's smaller, but not by a lot. Plus the current AF1 should be in the air for many years still.
They may look at a military conversion if there isn't a civilian airliner available.
I guess the one case where 4 engines might be useful is if somebody sneaks up with an IR-guided manpad near an airport. Such a small missile might just take out an engine without breaking up the entire plane.
Then again, in a war if you're operating AF1 anywhere close where hostile missiles are a concern, you're doing something wrong.