Got some 4" risers for my wife's bike and I can't get them to clamp properly. There are two nubs on the backside of the integrated speedo/top clamp that make contact with the riser. Can I just grind those nubs off?

 

Oh yeah! I wonder what was designed to screw into them. It is odd that your riser was machined fatter than the stock ones and hit them. It even looks like in your picture the riser was made to leave room. On my bike there is obviously a gap between these and the stock riser / clamp.

It looks like you could just cut them off.