Sort of.

Mixed brand has a lot of bugs, and the different ways you'd hook up more than 4 cards have a lot of drawbacks so you would have to plan this carefully. It would be easier and more flexible to get a 4 slot board for the AMD GPUs and use the NVIDIA ones in a different machine.

EDIT: note also that the 4 slot board does not need to populate all the PCIE lanes necessarily depending on your workload.