4U rackmount servers are available with up to 36 x 3.5" drive bays and accept full height expansion cards.
Servers designed to be fixed in a 19" Cabinet are typically described as rackmount, rack mounted, rack server, rack mountable, or occasionally simply shelf server. The height of the server is standardized as multiples of 1.75 inches (44.45 mm) or one rack unit or U. for instance a 4U Rackmount Server is 7 inches high. The depth of a 4U rackmount server is not so standard and can vary depending on the amount and type of processors, drive, bays, expansion cards and if you require redundant power supplies.
4U rackmount servers offer the maximum expansion potential with up to 8 full height and full length PCI or PCI Express slots. With up to 24 x 3.5" or 48 x 2.5" drive bays, or a mixture of 3.5" drive bays for optical and tape drives, 4U servers are a good choice where space is not an issue or at a premium cost, like a data center where you are being charged for space.
Different Power options are available for 4U Servers including DC power and redundant power options, lockable Doors are also available for various model 4U servers which can be used to stop unauthorized access to the drive bays and power switch.