Mate, if you want to have some more power from the 4k, i wouldnt head down the DOHC path.
Stick with a 4k. Your father being a mechanic will surely know a few tricks to improve performance.
Have a tinker and see how far you can push a 4k. Worst comes to worst, if/when she blows up, go grab another one, bolt it in and have another crack.
:(