[coreboot] KGPE-D16 - how to get PCI-e x16 on Slot 4?

According to the asus documentation it should be x16 if slot 2 is 
unoccupied, which it is.

I currently only have x8, I have tried changing the gpp config in 
devicetree.cb based on the SR5690 documentation but that results in a 
coreboot bootloop.

How many PCI-e slots can you use concurrently with PCI-e cards anyway? 
according to the manual it seems as though you can only do 4 although 
there are 5 physical slots.

- Thanks!