I think they're actually 4.25 or so (or was that the rears?).... Doesn't matter though, 4" will fit in either the front or rear and will sound as wonderfully shrill in either case.
Anyway, after listening to the stockers in my '88 for a week it didn't take me long to decide I needed something else. It's easy to make brackets for 6.5" speakers to fit the doors and you'll probably really like the difference. There's a writeup posted on another site... I'll see if I can find you a link so you get the general idea how the brackets look in the end.
I can't find the thread I'm thinking of, but here's one of someone who made a set. It'll give you a basic idea as to the desired shape.
http://www.supramania.com/forums/showthread.php?t=68425
Another pic:
http://img221.imageshack.us/my.php?i...scf14430dv.jpg
And some more detailed info:
http://www.supramania.com/forums/sho...6&postcount=32