4mill crank might help him ride a little better with the increased low and mid range due to the extra stroke. For most guys running a 4 mill we give up the extra low end for some more top end but if you kept it a mild port then it would make for a better low, and midrange.
I would put it together with a 4mill long rod crank, wiseco 795 series pistons and a cut head. If you space the cylinders off the case then your raising the port hights 2mm wich will in general shift the power to the top end.