Wait.... I have a new theory.

Four hitmen arrived in a refrigerated caterer's truck.

Two of them were dead already, and had been immediately frozen after they were killed.

The two living hit men dumped them in the ditch when no one was looking, and then hid in the woods while the dead guys were thawing out during the party.

Then, after opening fire, they escaped through the drainage ditch to the lake where a boat was witing for them.

Wait....here's a better one. Seriously, this time.

The two hitmen were either terminally ill already, promised a huge sum of money by Roth, or like Rocco who many believe went on a suicide mission of his own later, were attempting to redeem themselves in the eyes of their boss, Roth (maybe they were Moe Greene's bodyguards who were asleep at the switch when he was killed).

They entered the compound in a delivery truck for the party with the garrott marks already on their necks.

Then they hid in the woods until the drapes were opened and they opened fire.

Imediately after the shooting, the two hitmen then swallowed a quick-acting poison and jumped into the ditch, where they died moments later.

When they were found, no one suspected that they had poisoned themselves, because after seeing the marks on their neck it was assumed that they had been strangled.

I don't think there are any holes in that one, are there?