There's a roughly comparable example that happened in the LCS if I recall correctly. One of the premade teams had extreme Visa issues, and as such only one member of the regular team was able to attend an event. The team still played, but using four backup players.
You can definitely argue that the team was at a disadvantage. You can even feel sympathy for their situation. That said, I don't think it would be right to detract from the rewards of the team that beat them.
Such is matchmaking as we have it. It's not really our fault as players on the team with 5 if this happens, we were prepared for the game and we played our best. We should not be sold sort for that.
I'd lean towards reduced LP loss for the team with 4.
If you want a more drastic solution, I offer an alternative solution to AFK's/DC's, mid game player swaps. If a player doesn't join the game/come back in a certain time frame, then matchmaking will reach out to a sperate pool of players and ask them if they want to swap into the game as that champion. They don't know if their team is winning or losing, and they must own the champion. They can select their own runes/masteries/summoners, which replace the other players. The champion has all spent skill points returned, and all items are given a full refund. Champions or items that gain stacks over time maintain those stacks (the item must be bought back first). This gives a team a second chance at turning around the 5v4. Again, this is a drastic idea that will probably not be implemented, but before anyone brings it up, "it could be abused" can be applied to anything from a pencil to the champions selected in game.