There is one slight problem...
Lets say its United v Coventry in the play off final. Coventry know that if they win that game their "relegation" will be from the second tier they have just reached, to the third tier. If they lose, their relgation is from the third to the fourth tier, so fair enough, there's an incentiv for them.
However, there is absolutely no incentive for United as they will be promoted whatever happens. If Coventry win, they can't take up their second tier place and logic dictates that the play off final losers would take it. I suppose you could get round that by saying that if Coventry won, the 3rd bottom team in the second tier would be reprieved, but I could see United suing on that......