I love my 4 mil. Do it and you'll never go back. Call a builder and start picking there brain with info on your bike. They'll shoot you in the right direction. You can have them build it or have them just port it and you put out back together.
Now this is very important, there is a right way and a wrong way to build a 4 mil. I did both ways and wasted a few hundred bucks. So with a 4 mil you have more crank throw, 2 mil up and 2 nil down. With that your piston would smash the head. So to correct this you need to get custom cut domes or have your stock head cut. Back to that right way and wrong way, you get a spacer plate to raise EVERYTHING to keep the pistons from hitting. With everything that includes your port timings and now you have a lot more area to create a leak. I did this and it ran. But not to it's full potential. So for the best performance you have to get the jugs ported for the 4 mil and the head/ domes cut.