You're asking a scenerio which the amp has no control over to determine the load impedance, which is determined by the speakers themselves. If you had any circuit analysis background, you'd find out the question is moot.
Can a bridged amp at 4-ohm run an 8-ohm load? Yes. Can a bridged amp at 4-ohm run a 2-ohm load? Ideally yes, stably, no.
Basically your amp can handle a speaker load that is above 4 ohms, anything lower you'll fry your amp. In some cases there are amps out there that can handle a 3 ohm load (ie Arc Audio, US amps) but the mainstream amps normally can't. Make sure you select sub or subs that give you the end result of 4 ohms.