So tough to narrow it down to four players. You'd really need to highlight players who transcended all of their peers for an extended period of time, and maybe changed the game by taking it to a level that hadn't been seen before.
Mosconi certainly would be a good candidate.
After that, though, it gets murky. Lassiter? Maybe. But who in the late 60s on really dominated to the level that they would stand out? Buddy, Sigel, Varner, Earl, and Archer all had periods they were unstoppable, and that is excluding many others. Efren certainly changed the kicking game (keep in mind it had only recently switched to Texas Express and the US players had only been kicking five years) but did he really dominate his peers?
As for guys like Worst or Willis, well, we can only go off a little hearsay and I don't think it's reasonable to consider them when they didn't dominate competition for long periods.
After 2000 the game has become more international and it has become even harder to stand out. Filler is definitely at the height of pool and no one could do better than what he's done over the last 5-6 years, but I'm not sure that he has quite earned a spot on Mount Rushmore. But if he hasn't, who the heck has?
In the end I just don't see picking only four. There have just been too many great players. But if I had to pick with a gun to my head, I'd go Mosconi, Efren, Earl, and SVB. I hate that Filler isn't on this list but I ran out of room. Maybe he'd edge Efren out if I wasn't so damn sentimental. So I go back to not choosing.