4K HDMI video wall controllers is mostly used in multi-projector application.It is a four-channel video processor, which incorporates two 4K30 HDMI inputs and four HDMI with 1080P60 resolution outputs 

Basic application is to crop specific region from the source for each projector, and then set overlap pixels for edge blending (as in below picture).  If you have more than 4 projectors, you can cascade multiple controllers via HDMI input port.