And to the OP 4 mil means that the crank has a 4 millimeter longer stroke then a stock crank and then its the same concept for 7, 10, 14, 18, 20 mils.
also youll hear people say 4 mil long rod, that means that the rod is 115mm long a "regular" rod would be 110mm people say the the shorter rod delivers faster revs and the long rod creates less stress on the piston IMHO when I go 4 mil itll be a long rod.