A serval on pump fuel makes what a not so reliable high strung race gas stock cylinder does. I feel like if you had more cylinder life you should do a ported 4 mil, but since your cylinders are done any way, a new aftermarket monoblock is in order.
As far as the 4 mil, they are pretty friggin stout if you buy a quality crank. Definitely start reading up on the 421 serval set ups. There are guys keeping them on the more torquey side with smaller carbs and low end pipes, yet there are guys letting them breathe with bigger inframe pipes, carbs and port work.