In a previous life we were planning DC8 (four engines I know) freighters in/out of Easter and, whilst my brain cells are fading, because the freighters were to be of max weight full of fish heading to/from Japan (via a few intermediary stops) we couldn't possibly carry enought fuel to make it to an alternate from Easter.
As I recall, and as I say brain cells are fading, there was some rule that we carry something like 4 hours of alternate fuel and, basically, "hope for the best".