I bet you didn't know you could get an M4A1 handguard to reach 315 degrees F external temperature!
Apparently it isn't very good for the rifle though...
1) M4A1 barrel outlasted M16A2 barrel in test to destruction. First tested M4A1 barrel ran out of ammo before barrel burst. Second M4A1 barrel burst at 596 rounds in 3 minutes 32 seconds.
2) M16A2 barrel burst at 491 rounds in 2 minutes 49 seconds.
3) External barrel temperatures reached upwards of 1,500F and as high as 1,712F
4) The gas tube did not give before the barrel in any of the tests
5) Distinct change in sound and muzzle flash 30 rounds before barrel ruptured in all cases. Barrel had also drooped as much as an inch lower at the muzzle in all cases.
Final section of the PDF report also includes a list of all known accidents involving the M4A1 up until September 1996 and makes for interesting reading in itself.
I agree with Tweak, normal variance in manufacture and materials could account for the round count difference betweent the M16 and M4 barrels. You would have to run quite a few tests and apply a statistical analysis to the results to have any appreciable degree of confidence that there is a difference between a M4 and M16 barrels.