Rumor has it that Seinfeld portrayed all four of the temperament archetypes. If that's so, I could see George as SJ (certainly). Kramer could be the NT (ENTP?), which would leave Elaine and Jerry as either NF or SP.
Now, that doesn't mean Jerry was playing his type on the show, either, but...we usually do better playing who we are. And Jerry on the show and off the show is definitely introvert something. He's definitely uncomfortable on stage. But introverts can be in the public eye, too.
Reference? Found it here:
http://www.v-mgroup.com/workshops/index.cfm?PCID=364 (scroll down the page - last bullet note)