I finished third and the forth place finisher got more points than me. WTF?

It's based on your point coming into the game. You get points like you were playing a 1v1 against everyone, if that makes sense. So, the person in 4th must've been a lot lower rank than you coming in. Lower rank players are expected to finish lower than higher players, so they will gain huge rewards if the finish high, but lose not as much if they finish low. High ranked are opposite, with small gains but big losses