The front in-dash are a 4", but there are a "special application" speaker. Pioneer used to make a direct-fit that would pop right in, but I don't think they're available anymore (according to Crutchfield.com). If you try to fit most aftermarket 4" speakers in the space, you have to snip 2 of the ears off the basket, and bend one of the remaining ears.
You can put the most expensive speakers in the world in that spot and they will never sound good. There is no baffle for the speaker, no enclosure, so they will always sound "tinny". If you want better sound, you're gonna have to hack up your doors.
The rears in the Xcab (if the factory holes are there) are only 5". I have a really expensive set of 5-1/4" MB Quarts in mine now and they work very well.