When I shift the auto transmission into low gear while travelling at 25 mph the transmission will drop to low gear but then will feel like it has gone to neutral as there will no longer be any engine braking and engine will race up to 4000 rpm without any pressure on the accelerator.

 

First, why are you shifting into low gear, and how often are you doing this? My first reaction to this question is your Focus may have an underpowered engine that can barely fight it's way out of a wet paper bag. If an engine struggles to get a car moving, it's going to struggle to slow a car down. If your down shifting to 'save on the brakes', it's alot cheaper replacing brakes than repairing transmissions and over-reved engines.