Well, we originally designed it for 4 players, but in testing it, 3 players ended up more ideal, due to a combination of the map design and the balance structure of the game (once you have the Trickery, Forge & Well core, you pretty much have everything covered, including the Triple Supports). We decided to keep it as 4 just cause that's the same player count as the original. From what I've seen in the leaderboards, 3 or 4 can work well, just depends on playstyles and strategies.

Thanks for the answer! I also noticed gold is increased when playing 2-man.

The number of creeps per wave = 120, which is 4 times the regular amount. That then is split among the number of players, so 4 players = same gold amount you'd get normally, 3 players = x1.33 for everyone, 2 players = x2, and 1 player = x4.