I just bought a cover for my Cayenne, and it took a couple of weeks to get, I assume because it was custom made. It is a Covercraft NOAH cover, so they might make one for the GT4 especially if they get several orders. I would think the rear would only have to be enlarged from the other 981 Cayman covers to fit.
Prices depend on the size, and if you want mirror pockets. Mine was just over $300 but a Cayenne is a lot bigger than a Cayman, obviously. However, if they do a custom job I would expect it will be a little more too. I would guess a GT4 cover would be around $300 but I'm not in the business so take that for what its worth.
I managed to find a custom outdoor cover for the GT4 at California Car Cover - the cover fits very well and is secure, it covers most of the wheels too - we get quite a bit of wind in the parking garage and the cover stays put.
Just to be clear. I have a Cayman GTS with spoiler.
Received the GT3 cover today, although I thought GT3 is larger it term of mass but side mirror to side mirror is tight. Not the best fit like the regular cayman cover is but does the job for covering everything at once. The spoiler fit is comfy as I think GT3 spoiler is wider..