I am building a stock cube forged motor. Can I use a 4.00" crank or does the block need machine work?

 

a 4" crank won't yield stock cubic inches; it will be a stroker engine with more cubes than stock.

If you are asking if you need to machine the block for a 4" stroke crank, then the answer is yes, to an extent. The crank will fit the block and journals, but most stroker cranks will need to have have the buttom of the cylinders clearanced for the rod bolts (most usually hit slightly with stroker cranks). Other than that, it's always a good idea to align-bore the main journals to insure the crank spins freely.