Couldn't find the exact specifications for usable heatsinks, but I did find two reviews that used 153mm and 158mm HSFs in their builds.
So I would assume you have the freedom to use either cooler since the NH-U12P is 158mm as well.
If you're not overclocking (3.7GHz is about stock for the FX-4100 right?) then you should be fine with the NH-U9B.
Please read through the thread thoroughly before responding.
Nobody wants to read a wall of text, formatting is your friend.
i5 3570k | 8GB G.Skill 1600MHz | R9 290 | SeaSonic 750X