what are the downpayment requirements for a 3 or 4 family? Someone was telling me that minimum 20%-25% is required for these, even if primary. If that's the case, then why not just buy as an investment property? What's the point of PMI?

Depends on the bank where you're getting your mortgage. These days most banks won't consider anything below 20%.

for 3 family is one thing and for 4 family is something else. a 3 family is still Freddie/Fannie, 4 is not.

Don't forget FHFA conforming loan limits: single family: $625,500; 2-family: $800,775; 3-family: $967,950; and 4-family: $1,202,925.