... rear speakers should be used to provide the 'fill' in your sound with the front speakers + tweeters for the better part. It's a little tough in the small cabin of an X ...
Make your own kick speakers boxes for the fronts, 4", 5.25 or 6". Or cut the perfectly good interior door panels. Tweeters should be at ear level.
As mentioned, the rear 4x6M, have to have a shallow mounting depth because of the fuel tank clearance. Look for a top mounting depth of 1 3/4" (Alpine) or 1 5/8" (Pioneer). Anything more like 1 15/16" (Infinity Reference) is likely to protrude by a cluster of gnat hairs. Many speaker vendors often describe the shallow mounting speakers as plate speakers.
Alpine, Pioneer, Infinity, in no particular order of preference. Just look at the top mounting depth ... it will still be a tight fit on the fuel tank side anyway ...
When shopping for speakers, check out the top mounting depth, RMS peak power (what is the head unit output?), frequency response and, sensitivity (90+ dB or higher is better). I look at the top mount depth first, then sensitivity, then RMS. Narrows down the choices but you'll be better for the comparison rather than price.
regardless, you should consider an amp in the mix, as the head unit won't have the power.
Oh it's fun!