I might be hallucinating, but IIRC this is now dependent on PCIe revision. More recent revisions have removed the slot power limits line, and so it works out that in theory a x4 slot (or any other size) can have a 75W load. IIRC this is stated in the middle of a giant section on up/down plugging.
(but its also displayed in the pcie PowerLimit capabilities you can see with lspci)
(but its also displayed in the pcie PowerLimit capabilities you can see with lspci)