The rears should be flat on inside as well. There are a lot of quad boards around cos of the interest in them, and some work and some don't.
For kiting if the rears are symetric it makes the boards track too much and you loose the advantage of being able to ride on rail going upwind etc.
To get the full advantage of a quad the rears should be forward closer to the side fins than a thruster and all fins should be flat on inside.
The only time you might use symetric fins would be if they were quite small trailing fins and the side fins were a lot bigger.
This has been my experience anyway.