Streamer Stage 1 4NT Set Up

[Gazzas01]

I recently took my bass to get set up and was told that the neck was bowed and the truss rod was not working correctly and would need replaced. Having had a look myself and done some research, I am led to believe that the neck is supposed to be slightly concave and the truss rod only adjusts 1 way. I would like to lower the action on my bass, as it is about 4mm on the E string at the 24th fret at the moment. Any help or suggestions would be greatly appreciated. Thanks