I would say, leave th standard's in there, cap them, use them as rear fillers with them turned right down... no need for speakers back there anyway, what do you care what it sounds like in the back... as long as there is a party up front

That deck work Joe?

also, to answer ur questions, you can buy 4" aftermarket speakers.. if thats ur thing, you'll probably find you can fit 5&1/4" in there too, without too much headache. depth will not be a problem