The 400-series has two emitters, for twice the output on full power and a slightly larger visible target. The minimum output on steady-burn mode has been reduced a little, although I still wouldn't use it in any mode on an MUP. Another big difference is the mounting system and button location.

I'm pretty satisfied with my 140. If I rode in dense fog a lot, I'd strongly consider upgrading to the 400... fog is bad news.