From what I understand the minimum spawn was raised to 3-4 players now right? Do you get the exp&drop benefits from running solo as if there were 4 players in the game, or does it just effect the monster hp? Basically wanna know if I'm wasting my time loading my exp/farm games to 4 players.

Only hp, no extra exp or loot when running solo.

As things stand right now players 1-3 have 300% base monster health. It increases 50% per player above 3.