It's important to realise that with English grammar, as for grammar in many (maybe most?) languages, there is not a single defined interpretation for every possible construct.
Depending on context, I would suggest that there were two valid interpretations and one invalid interpretation:
- Possible: four people, some American and some British. Context: talking about "westerners" on a bus in rural Asia.
- Possible: four people, all of whom have dual citizenship of Britain and America. Context: discussion of dual citizens at immigration control.
- Not possible: four British people, four American people, making eight in total.
In this case I would say that this comes down to the fact that there is a number involved and to its placement, however the key point is that with only a slight variation in the sentence, the "and" could be interpreted differently.
This is why, as one of the commenters pointed out, this sentence is an example of poor construction, in which (without context) there is no single right answer.
Off-topic aside: as part of a software design job, I once did user research on whether people interpreted "a and b or c" as "(a and b) or c" or "a and (b or c)", and other such permutations. The results showed that:
In almost no examples did everyone agree
Where there was general agreement, it largely came down to assumptions around context, rather than the grammar itself