There is also the Clement Street option, inaccessible by BART and nearly impossible to do within a 4 hour layover, but there's some amazing values there for Dim Sum (you won't believe how much dim sum $5-10 will get you in some of those hole-in-the-wall places).

My recommendations are Gourmet Dim Sum and Good Luck Dim Sum, both within a block of each other.

Disclaimer: this is fast-food, not sit-down fare (although I would put either of the places mentioned above up against any dim sum restaurant in the San Diego area, and I think Clement Street wins hands-down).