Out of curiosity, why are you looking for it? IF has always struck me as a fairly irrelevant statistic (a lot of vital journals get shafted thanks to it, merely because they're in a niche few researchers care about).
Seems like an IF of 4 for a medical journal is pretty easy. Most of the top ones on the good professor's link were to medical/medicine-related fields (only one ranked higher than Nature was in something else, for example). Makes sense, too--there are, to put it bluntly, a lot more doctors than there are researchers in other fields (meaning other individual fields, so like "There are more doctors than mathematicians", in case that's not clear).