4x2, pronounced 4 by 2, is a slang name for a Jew. Not sure if its 'official' Cockney rhyming slang, but it does rhyme. Also sometimes just the 4 is used, as again in Cockney rhyming slang the phrase can be shortened to just the first word.
"Typical tight-fisted 4x2"
"Of course he didn't get a round in, he's a 4!"