Retopoflow 3 had a whole symmetry system that was really just the mirror modifier under the hood.
In Retopoflow 4, you have full access to the mirror modifier directly and do not need to do anything extra for that to work with the Retopoflow tools.
It is important to note that, while the retopology overlay is enabled, Blender only shows the result of modifiers if you enable the On Cage (mesh triangle) option in the modifier’s header. In all other cases it is highly recommended to work with this off.
Working with Blender’s Edit Mode Symmetry is not yet supported, as unfortunately it is rather broken and many regular operations in Blender are not yet supported. We hope to help improve this for all Blender users in the future.