The reason that I run 4 is so I don't have to split the screens so much that they get too small to see.
Here is the setup that I typically run.
HDS12 split screen with Chart and sonar. Or if I'm running a new lake or following trails of boat lanes, I'll split the Chart into 2 screens, then zoom in on one and leave the other out to see a bigger pic of the lake.
HDS9 split screen Structure scan down imaging and side imaging.
HDS9 on a raised mount center over my tm pedal. I run a split screen showing chart and down imaging.
HDS5 flush mounted displays sonar.
This works for me but with the 4 graphs you can split up the screens however works best for you.
I hope this helps and if you have any other questions just send me a PM.
Phil 4:13 "I can do all things through Christ who strengthens me"