Ensure that it is indeed a shutdown, I am thinking that its just installing the 4th card and normally will do a blank screen while installing it and it take a few minutes. Also check your Risers, if you have extras try to replace them.
Also got this info from MSI website with this particular board..
I wonder if its the cause.
** The PCI_E2/ PCI_E5 slot will be unavailable when an expansion card has been installed in the PCI_E3/ PCI1 slot.
https://www.msi.com/Motherboard/Z270-PC-MATE/SpecificationNOW I get 4 gpu without driver but when I install Nvidia driver I get 3 gpus working good and one with errors 43 on pcie e-3... the card works well if I install just it...
I don't think msi advice has effect on it but I'm not sure