yes you can replace the knob with no lock nut. look around on the net and you will find many billet alluminum shift knobs made specifically for the s4 and good place to go for anything is
www.performancecenter.com they can help you with alot of things. what ever you do dont get a universal i got one and the little screws that hold the plastic peice in messed up my threading. so i recomend going with a billet alluminum knob