Well, with a 4mm stroker crank, you are basically pushing the piston up 2mm higher out the top, and pulling it 2mm further down the hole. But the stroker also affects piston speed, and the duration at which the piston stays at the top and bottom. what pistons are you using? standard? or 795 series? plate under the cylinders or cut domes? For me, when I build a 4mil motor, I dont use the plate, and just port accordingly, and run domes that are cut to clear a 795 series piston and I use longrods.