While for the most part you are going to be putting together server racks that are going to remain in the same location over their entire lifetime, there is going to come a time when you may be asked to build a mobile server system. When this happens you are likely to find that using a 4 post rack that has wheels mounted on the base.
The 4 Post Rack Is More Structurally Rigid
One of the biggest reasons that most IT techs will choose a 4 post rack over a 2 post design is that these racks are far more structurally rigid. This means that you can load them down with all of the servers and support equipment your client is going to need and they will still roll around as necessary without collapsing or falling apart, ensuring that the equipment is not going to be damaged.