4x6s alone are not going to work well with two SA-8s. You'll have 0 midbass, and you'll be sad. Even if you have to resort to a pair of 6x9s in boxes you should look at adding speakers to cover the gap between the 8s and the 4x6s.
My best options for 4x6 are Infinity Kappa 462.9cfp. This is basically a 4" component set on a 4x6 plate. I also have Pioneer TS-A4675R.