ok 3.5 speaker is 3.5" round and 4" speaker is 4" round think about that, they are named after their size so without cutting your not gonna get a 4" speaker to sit properly in the 3.5 hole its like tryen to fit sumthing square in a round hole not gonna go in and sit properly
I'll say once again, putting 4's instead of 3.5's is pointless. They are probably going to be close in power handling and they should be crossed over just about the same, so there will be no difference in sound. Now if you're going to do door pods or kick panels, that's when size matters.
i just bought some 4" for the front and they work awesome. now i just have to fit them in there. if u find an easy way to do it, let me know. and if i find an easy way to do it, ill let you know. i think the left side is gonna be a little harder.
p.s i got the 4's instead of 3.5's because the brand that i got ( Dragster) doesn't come in 3.5's.
I got my 4" 's in by using a dremel to grind away the plastic. (melts like butter). then all you have to do is pre drill the holes for mounting. That's it. The only real downside with the 4" 's is that the covers may not fit over the tweeters. (depending on your speaker)