I've got to take 12" inches off of a 10' run of concrete slab that's
about 4" thick. I was thinking about using a small rotary hammer to
either drill a bunch of holes and crack it off with a sledge, or use
the hammer only mode with a chisel. Am I going to be asking too much
of the tool? It's a Makita 1" D-Handle Rotary Hammer "Put Bull" model
HR2455 that draws 7amps.

You SHOULD use a concrete saw. If you're going to use
a hammer and or drill, take 2 feet off, and then cast
a new edge.