Render only part of screen

The bottom quarter of my screen is always covered by ui is it possible to stop rendering the 3D world behind it. Like a mask or something?

 

Draw an opaque rectangle as close to the front plane of the camera's frustum as it possible. Presumably, this can increase performance, because, for geometry in the back, only rasterization and z test operations will be executed. But in practice, the performance gain would be miniscule.

add
If the screen is divided as "||||", and not as "+", then just change the rect in the camera settings.