I don't think you can get more than 3. The problem with that is that you can find something wrong on most plays if you look hard enough. So I lose my complete pass challenge, but hang on, on the other side of the field, there was DPI! I don't like the slope you start sliding down if I'm allowed to point out 4 problems on a challenge. Really though, if we're going to stick with replay, there's no reason rule anyone down on potential fumbles, ever. Make a judgement call on the fumble first, because ruling down by contact if it's NOT a fumble is a lot easier to get right upon review, than deciding who would have picked it up in an alternate reality.