You have received several BS answers, a 7 1/2 will cut a 4x4 after 1 pass, rotate it 90 degrees and complete the cut. If you get him a decent saw he will love it as it is the most versatile avail. Grt him a nice one, I'd suggest a Porter Cable or Milwaukee, these are 2 of the best, abot $125.
That being said he may mean a chop saw or cut off saw which is not as versatile as a circular saw. The 10" saw is the most common, I dont even know is you can get an 8". A 10" chop saw easily does a 4x4 in one cut.
You need to know which type he prefers.
Oh, and PS: I have cut many perfect 4x4 cuts with a circular saw.