I've never heard of a 4.00 inch stroke for a big block. 3.90, 4.15 and 4.25 are all very common but 4.00 is a new one on me. If the heads are off you can measure the stroke fairly easily by using a ruler on the top of the piston. If the engine is all together and it runs good then it doesn't really matter what the stroke is. If you're trying to buy pistons then you'll need to know what you're dealing with.
