I've definitely heard people say something like your example 4. Whether it is grammatically correct, I don't know. To me, "you had to have seen it" makes more sense to me in that context, but I notice that many people seem to shy away from saying it in that way.
To me, "you had to see it" would mean that you were required to see it, either by internal urges or external constraints. For example, an irate parent who finds out that his child has sneaked in to see an adult-oriented film: "You knew it was wrong, but you just had to see it, didn't you?" In this case, only "had to see it" works for me.
To carry the example further, let's say the child says, "I didn't see any adult film." The parent replies, "Look, son, I have witnesses who saw you go into the theatre and not come out for two hours. You had/have to have seen it." To me, this sentence calls for "had/have to have seen"; "had to see" sounds wrong to me here.
Would "the Frenchman must have outweighed him" communicate the same as the original " the Frenchman had to outweigh him"? I guess "must have" would make what she thought as a logical conclusion, instead of an established fact that "had to" suggests.
Example: William’s mussed hair and blackened eye, not to mention the blood leaking from the Frenchman’s nose, suggested that William had put up a valiant struggle, but the Frenchman had to outweigh him by three or four stone and had full use of all his limbs.
Source: Timepiece, Albano, Heather
Background: When she came back from relieving herself in a bush, she found that her companions (Max and William) were surrounded by a group of French soldiers with muskets pointing nonchalantly at their heads. She stopped her footsteps and held still behind a tree, straining her ears to make out what the soldiers and Max were talking in French.