Unfortunately, if you are placed into a Low Priority Queue for leaving this match, per Riot's Support Page:
CAN YOU TAKE ME OUT OF LOW PRIORITY QUEUE?
No. We will not take your account out of low priority queue under any circumstances.
If you choose to leave the match for any reason, then you will be subject to the consequences of that leaving; which includes the initial warnings and subsequent waiting periods of the Low Priority Queue. Had it been due to an error on the part of Riot's Server, they might have considered it, if it was for the very first punishment level. But when you come here and admit that you are willingly abandoning the match simply because you don't want to wait to see if the 5th player connects or play as a 4v5... those sorts of actions are exactly why the LPQ exists. To keep players from doing what you've just done. A 4v5 match is winnable. And the 5th player may have connected. But now, you'll never know. And at best, if he comes back it's a 4v5 still. At worst, you just made it a 3v5 for those other innocent players on your team.
Riot is looking to discourage this sort of behavior; thus the introduction of the Low Priority Queue, with it's escalating tiers of queue waiting penalties. The more a player leaves matches, the longer they have to wait.
