Jerry - ISTP
Elaine - ESTJ
Kramer - ENTP (or possibly ENFP)
George - George is a weird one, because the character is based on Larry David, an INFJ, but Jason Alexander plays him a little differently. So I think he's some sort of a hybrid between INFJ and maybe ESFP. If I had to stick one type on him I'd say INFJ since they tend to display a variety of personality traits anyway.