Slap occurs when the pistons can move around too much in the bore, the 4wd has an offset gudgeon pin hole bore which cuts down on this, but if the pistons/bores are worn you can still get slap.

A correctly rebuilt 4wd motor on Mahle pistons with the correct piston to bore clearance (around 2 thou off the top of my head) wont have any slap at all even from cold.