As I got my camera with a 35/1.4 and the 90/2.0 later, I would recommend the 35/1.4 or 35/2 as well. I usually use one camera with a .72 viewfinder and the 35 mm and a "back-up" M3 or S/M with a 50 mm, but 90+% of the shots will be taken with the 35 mm.
When I switched to Leica M the improvement was using slower b/w films with slower shooter speeds and wide open aperture. Like using 400 speed film instead of 3200.
So a slow lens like /4 would not give me that advantage.

35 and 90 is an excellent allround kit.

Wolfram