The four High Lords of the Truckers are Big Bang, Gravity, Death, and Betsy. The role and authority of the High Lords is largely unknown- their only certain role is presiding over Trucker Trials.
When presiding over Trucker Trials the High Lords sit on the Trucker Thrones in the Great Multidimensional Truckstop. These are super-comfy truck seats, complete with cup holders held high in the air by giant stands. Judging by the fact that Betsy has to sit on a box, the Trucker Thrones seem to be unalterable, either through some mystical power or the reverance in which the truckers hold them.
The number assigned to each High Lord appears to be based on the order joined. Big Bang cast everything into existence, Gravity started pulling it together, life began, Death followed, and eventually life progressed to the point when Betsy was born.
Qualification for a High Lordship appears to be reserved for entities that move lots of things. Big Bang moves all things from the starting point, Gravity brings all things together, Death brings souls to the next world, and Betsy does a load of impressive shit that really shouldn't be physically possible.