Before you buy your servers make sure the "4U" space is really all about space and not about power consumption. At Hurricane Electric in bay area California yo

Before you buy your servers make sure the "4U" space is really all about space and not about power consumption. At Hurricane Electric in bay area California you can rent sub-rack space that could hold, say, 8U. However, if you try to fill all that space with SuperMicro high-density 1U systems you'll far outstrip the power they're willing to supply for the rental rate. I don't know about your colo's policies but please double-check.