I think I know the situation the OP is referring to where an ALT CST* of 4000 is coded into the STAR but is not actually required when approaching the ILS from the east. If a pilot continues in managed descent then the aircraft will level off at the CST prior to intercepting the G/S instead of continuing the descent to 3000' which is the cleared level. In my view if ATC have cleared you to a specific level then that's the level you should descend to. They are not aware of how the FMS is coded and are not expecting you to level off. If you need to slow down to configure then select V/S so that the descent to the cleared level is maintained.