I thought that if a unit clips, then it is treated just like a charge and therefore you have to bring the maximum number of models on both sides into combat. Thus Red's move in 4.3 would not be allowed, as he could have one more model in combat.
In our own games, certainly, no-one would allow 4.3. As I have often said, why would the one guy who could get into combat say 'NO, I'll not bother. I'll just stay here"? Or, perhaps better put, why would his comrades and officers allow him to do so?
References (though not, annoyingly, directly relevant, but rather 'informative' regarding the 'spirit' of the game and similar situations):
BRB p.20: " ... As many models as possible from the two units must be brought into base to base contact." // "... what would happen in a real battle is that the warriors of the two units would quickly move to attack their enemies ..."
BRB p.58 Pursuit into a new enemy: "The charging unit must wheel and close the door in such a way as to maximise contact, as they would with a normal charge."
BRB p.22 Unusual situations: " ... in some situations the enemy might have to close the door with the chargers instead ... as they would do in reality."
Indeed, with all these rules in mind one might argue that if blue had charged, then the clipping should never have been allowed - either red should have moved, or both units wheeled a little, or blue should have wheeled onto red's flank.
The GM (if you have one) would be a busy person!
Of course, I put all this here in the hope you can point to the actual rules that deal with the clipping situation in 4.1. I can't find the rules on clipping in BRB 6th - 8th ed., nor in any errata. (I seem to have, however, a memory of seeing some clipping rules somewhere, once, back in the mists of time. I have been playing since 1st ed!.)