By watching runs I couldn't understand what was happening in Phase 4 to get the 3200 points - but then I looked up the gamefaq on prizebombs where it explains the golden hammer and the formula.

I'm able to reproduce this now and get the golden hammer although it's still interesting how the prizebomb is manipulated so that it is considered to be blown up 3rd even though it is hit second? I'm assuming the extra swing taken at the 1st bomb is considered another bomb hit?

Your assumption is correct.

The prize counter doesn't care if three bombs actually explode. It counts hammer swings on tiles that have or used to have bombs.

so is there a technique on how to get the golden hammer?