A cat 3 meter on a portable generator is fine until there is a lightning strike. Of course, using a cat 4 won't guarantee you won't get hurt, but if it translates to a voltage surge, a cat 4 meter offers better protection than a cat 3.
The idea is that, in the event of a lightning strike, if it occurs far enough away that it's not a direct hit then you're better of with a higher category of meter. And being that a portable genny is typically used outside, the odds are higher.
Here's the other thing: a cat 4 meter doesn't cost much more than a cat 3, so why not err to the safe side? Strictly speaking, according to UL 1244, a cat 4 meter should be used on distribution level equipment, which includes portable gennys. But people tend to go with their experience and until they experience catastrophe they believe what they've been doing is fine. That's why I say that experience doesn't always teach the right lessons.