The first I know of appeared yesterday: Samsung Galaxy S4 Review
It's fairly short, and rather perfunctory on some important points (e.g. battery life). They don't say whether it's a final or a pre-production unit, though they do detail the software build the unit had. So a long way from the last word in reviews IMO, but it's there for those who want to read it.
They've given it the exact same "score" they gave the SIII last year.