Copper will kill inverts at levels below where you can test for it's presence.
An equilibrium will exist in your tank. You can remove copper from the water column using polyfilter and/or other products designed for removing copper. Your rock and substrate (not silicone) will be consistently leaching copper for undetermined period of time.
Even things are going well you don't know if your tank is time-bomb. Small events could shift the equilibrium in the tank releasing copper, such as an increase in temperature or potentially a disruption in your sand bed releasing copper in the substrate that was buried deeper.
On a side note 4" is not a good substrate depth. Too deep to have the benefits of shallow sandbed, and not deep enough to have the benefits of a DSB.