addition

Hello, all! This is my first post, so thanks for having me. I am chasing a hot water union between original galvanized piping and a copper addition. Unfortunately, it appears to disappear into a concrete slab. There are markings on the slab and would like to have some idea what they might mean...

I'm extending the back of my house out 14 feet (the current room is 12x23 and has 3 registers) it's a one-story open concept house. I need to disconnect two of the original three registers and 4" pipes. Can I simply add a few extra lengths 4" pipe or do I have to use a larger diameter pipe...