I have fitted 4wd pistons into my 2wd engine. The compression height is the same for both piston.

As cng1 stated the 4wd pistons do have a larger dish. Once everything was measured up it worked out that the 4wd piston with a 2wd head has a compression ratio of approx 7.6 to 1. (Exact figure depends on head gasket thickness and how much of a skim the head has had).