They’re all grammatically correct however they vary slightly in their nuance. Sentence 1 - is expressing a factual real result Sentence 2 - is purpose, “could” hear him Sentence 3 - purpose again but lack of certainty “might hear him” Sentence 4 - is grammatically unnatural but theoretically correct, unlikely to be used in natural speech or writing if it were written “he spoke loudly because everyone should be able to him” this is slightly more natural. Hope this helps 😊