So you really want a fourth bike.

My first question is, Why do you want another bike? Do you r want another biker just to not have disc brakes? Do you want a bike with a slightly different ride, maybe more comfortable for longer rides or lighter for climbing? You seem to be looking at a bike almost identical; to the one you have, which makes me wonder if, having two almost identical bikes, you wouldn't ride one almost exclusively anyway.

I am not making value judgments here ... in fact, I am a strong supporter of n+1. It is just hard to offer sound advice based on so little information.