Rackmount Case Power Supply

I looked at rackmount cases a few months ago for a possible project. A 4U rackmount case can easily fit a standard atx power supply but there is a catch. Rackmount chassis come in standard sizes. The standard width is about 17 inches overall. That includes the left and right mounting flanges on the front panel. The flanges are used to secure the chassis to a rack. The interior of the chassis is not as wide. Most of the ones I looked at were designed to mount a power supply so that it is resting on it's side. This done to make room for motherboards. At a height of 4U you should be okay.

I would suggest waiting until the chassis arrives so you can take a good look at it and take measurements for the power supply. If for some reason a standard atx power supply will not fit, then you could get a rackmount chassis psu available from online vendors.