I overheated (melted) the 4-8 Pin EPS with only the 4pin plugged in on my OC'ed Xeon i7 at like 4-4.2ghz but shouldn't be a problem on a mAtx build! Running Athlon II X2 rig in sig with only 4pin and have OC'ed it.

Only used 4pin for my Q9550@4ghz too for a year no problems!

So you will be fine with 4pinner