Input/Output Levels

I am having an issue with input/output levels. I am using Cubase to mix live audio recordings with 2 different sets of microphones each recording a mono file, for a total of 4 mono files which are panned left or right.

when I activate either set of microphones (inputs 1&2 or 3&4) the output levels are comparable. However, when I activate all 4 input sources (2 panned left, 2 panned right) the output signal is significantly greater on right (or left) portion.

Any help would be appreciated. I previously used Cubase 6 and upgraded to 8 in hopes that would correct the issue but did not.