This one has me stumped. I've written a monte-carlo simulator, and probability of rolling such a combination is something like 50.1%
My thinking is that there are C(6, 4)=15 ways to have 4 unique numbers, and for each of these there are C(6+4-1, 6)=84 ways to have 6 dice land on these numbers. but this gives 1260 ways, which is completely unreasonable when there are C(6+6-1,6)=462 ways to roll 6 dice.
I just can't wrap my brain around this one. Any ideas?