The 4mm will be a much cheaper route to go. 10mm cranks are about 2x the cost, and you gotta pay to get your cases trenched. Additionally, if you plan on running your stock cylinders, a 4mm crank will be much more reliable and will develop more HP/$ spent.
I've had 4 different 4mm motors now (2 cubs and 2 stock cylinders) and they are a very snorty motor for a stock frame bike.
Do you have to trench the cases with a 4 mm?