Load rating and wheel base to start with.
Lots of details are different. The frame of a 4100 is made of lighter stuff, the same as a 1-ton. The standard front axle on a 4000 series is lighter than that on a 6000 series. And the list goes on.
If you really want to dig in and understand the differences I'd encourage you to spend some time at the
'49 Chevy Restoration Pack found at the link.
