What is the difference between an A3 and an A4 upper receiver?

nothing

commercial market calls it A3, military calls it A4

Try Google?  
It might be able to answer the M4 cut answer as well

The commercial market jumped the gun, introducing the A3 upper before the .mil had adopted an M16A3.  They just sort of assumed that the A3 would be a flat top.  Turned out the .mil's A3 is identical to an A2 except for having full auto instead of burst.  The M16A4 was the one that got the flat top upper.  So you've got some companies going by the original commercial term, while others are using the military's term.  But on the commercial market, they're the same thing.

This is correct.

Thanks for the insight.