I understand what you're saying. Without fish in the tank, the only real ammonia being produced in from organic die off (breakdown) from live rock. After 4 weeks, I would think that you have some established beneificial bacteria in their depending on the amount of life that was on the live rock orginally added. Adding more live rock, in my mind, would only enhance the ability of the existing beneficial bacteria by providing more "food" (ammonia/waste). Depending on the amount of rock, I don't think you will be doing any harm or putting the current inverts in harms way by adding more.......now I'm not talking rock that has nasty junk all over it and has been sitting on someone's back porch for the summer.