To access VIC-IV features, one needs to put the values 71 and 83 into the KEY register (53295).
Those of you who try this from BASIC10 are in for a nasty surprise, because between the first and the second poke, the VIC gets thrown into VIC-II mode and hides all of its extra registers. But since the C65 interrupt depends on those registers being there, a freeze or crash usually follows.
Here's a little routine that you can use to switch to VIC-IV mode without crashing the system:
Once there, you can use BASIC10 to play around with some of the VIC-IV's advanced features, like the sprite bitplane mode: