3- and 4-holed dehumidifiers have three and four holes, respectively. By having four holes, it is possible to have separate airflows for regeneration and process air. For 3-holed units, there’s only one inlet, shared by both the regeneration and the process air. This means that one fan can be used to drive both airflows. It also means that the dehumidifier will create a slight vacuum in the room where it is installed and thus increase infiltration a little bit.
Main advantages of the 3-holed solution are that it is a cheaper unit and easier to move around and use in temporary installations (only one duct to leave the room). The main disadvantage of a 3-holed setup is that the slight vacuum will increase infiltration and result in a higher moisture load and thus decrease efficiency.
The main advantage of a 4-holed setup is that the efficiency is kept higher as the “disturbance” in the room will be kept on a minimum. Main disadvantages of the 4-holed setup are that it requires two fans (bigger unit and higher cost) and two ducts to leave the room (regeneration in and out).
In general: 3-holed setup is recommended for temporary installations and 4-holed setup is recommended for stationary installations.