The front's are 6x4, i put some
JBL GTO-6427 6x4 Speakers in mine as i didn't want to cut the metal work to get bigger speakers to fit. They are not bad speakers which only need a little trim of the plastic to fit them into the existing speaker pods. I havent got round to sorting the rear ones out yet, so can't help you there mate, but i'm sure someone else know's of a good replacement for the rears.
R32Harv