A booster will enable your membranes to work at a more optimal level so it will decrease your water usage (you'll prodeuce more pure from the incoming water than without one)
It will also lower your pure tds
You should be able to adjust your pure/waste ratio already on a 4040 though, there should be a valve/tap you can adjust. The more you put to waste, the slower you produce pure but you'll have a lower tds and visa versa (i think its that way around!)