I wonder if that really adds much information value in the grand scheme of things. Yelp is built for ratings and reviews and I can't see myself making the extra trip to Foursquare to see how its users rate something when Yelp will be far more useful.

But what 4sq has that Yelp doesn't is number of visits (per user) and time-relation of visits between places. Instead of ratings, a metric unique to 4sq would be repeat visits (cleaned in such a way to eliminate superficial checkins) and where people go immediately before and after visiting a given place. I would rather see that kind of behavioral data leveraged more than have one more place where people rate things