I am going to be throwing a lot of suggestions/questions your way so try and see what works. Hopefully something works.
Do you have 4G enabled? What OS are you using? Have you tried moving your power supplies around so that the 4th GPU would be connected to the 1200 PSU for trial 1 and then connect the 4th GPU to 750 PSU, if that is your setup (Just checking if you have delegated enough power to the GPUs)... What risers are you using? Does each riser have its own individual power cable (make sure not to have one power cable powering more than one riser at a time)? Do you have both 8 pins connected to each GPU (I am not sure what pin connectors MSI has)? Have you updated your BIOS and cards to the latest version? Have you checked Device Manager to see if there is a driver issue or hardware issue?