It's a difficult certainty to put into numbers. The infinite number of worlds spanning across an infinite number of parallel and divergent dimensions makes the odds essentially zero, while at the same time being infinitely large. Every four days, he has the same chance as any other individual of arriving here. It's more than possible that we would all die of old age long before a warden with control over spacetime saw fit to bring him here.
It would be easy to observe that a 'clustering' effect seems to take place, where metaphorical flood gates open after one individual has been brought here from a given world, but to be perfectly frank that 'effect' is as illusory as any other pattern the human mind picks out of chaos.
[ pause, and then he adds cheerfully ] Not that we're dealing in pure chaos here.