I see the specs say it works with lipo but the input voltage is between 4.8-7.4V lipos will have 8.4v so is that what the built in bec is for?

you are correct

NO. BEC (battery eliminator circuit) is used to power your servo and receiver. It does nothing to regulate the votage of the battery. Without a bec you would need a seperate power source for servos and the receiver.....the 4 AA's used in electrics way back when.

7.2v nimh packs aren't 7.2v either, charge one and put a volt meter on it, it should be over or around 8-9v.