With a 4k composition you render 4x the pixels with every layer (on 4k layer sizes) If your clips are 1080p that is a waste of resources.
Routing 1080p layers directly to slices saves a lot of processing, especially if you also run effects.
Setting the layers to be 1080p in the layer video settings, and aligning those up in a 4 composition is something in between, with the benefit, that you see what your output is.
Software developer, Sound Engineer,
Control Your show with ”Enter” - multiple Resolume servers at once - SMPTE/MTC column launch
try for free: http://programs.palffyzoltan.hu