Looks fairly normal, especially if you didn't have the rings properly broken in and set after the rebuild. A little Easy Off oven cleaner will take that right off the head, but DON'T leave it on, or else it can etch the aluminum. Just spray the cleaner off, let it bubble a bit, use a toothbrush to brush it a little bit, maybe a flexible paint scraper (nylon), they squirt it off with high pressure water or just plain old hose water with a good nozzle... works like a charm.

I'm guessing that when you put a straight edge to the head it'll still be flat, and the valves still seal. If that is the case, I would pull the valve springs, then re-lap the valves with lapping compound and a lapping tool (suction cup on the end of a stick.)

When you pull the block apart, you should probably leave the carbon on the sides of the pistons above the compression rings alone, and perhaps the carbon ring at the top of the cylinders if you can help it. There is no problem with taking the carbon off the tops of the pistons though.

Hmmm... with only 4K on that rebuild something wasn't done right... some corner was cut, or not reassembled correctly. How much oil was it burning?