4x6 speakers are not terrible, but Im sorry they are simply inferior to 6.5 component sets of equal quality. 4x6's exist to fit tight spaces that a larger (and better) driver cannot, period. I do think they are a good choice for many people, people who want a simple system and dont have the ability to manufacture locations to mount bigger drivers. But, to argue that 4x6's are somehow better drivers than a 6.5 in general terms (like they play higher) is a losing arguement.
As for front speakers. I just replaced my functioning stock 4x6 speakers two weeks ago. $49 for a pair of Alpine 4x6. They come with two trim adapters (use the thicker set)