If it were me, I would set up the S4s by themselves and if they did everything I need, I would not complicate things with extenders or routers. IMO, the less the better. If I needed more coverage, I would prefer to add more S4s as they are designed to work together. Although many extenders and routers are mesh rated and may work fine with the S4s, mixing different models with each other sometimes can be problematic. Keep it simple. Note that I do not have a mesh system and do have a TP Link extender in use with my Archer C8. When it comes time to replace, I am looking at a Mesh system.
