You might only need 4 slots because:
But you might have a reason for 5 slots, say a really thick radiator.
- People only ever use dual radiators for a full loop, including the CPU and the GPU. This means that there is a GPU waterblock, which is around 1 slot or more. So, you have 50mm to 60mm for the rad+fans, which is definitely enough.
- I've never seen any GPU that's >4 slots
There has to be a reason why people would choose this over the C26!
Supposing I get the case down to 4 slots, and then have 10mm extra for more space to more easily install the motherboard into. 10mm is only 1/2 slot, but enough space that could make a difference with radiators.
A GPU with a full cover waterblock takes up 1 slot leaving 3 slots, which are ~57mm of space. The extra 10mm will make 67mm. A standard 30mm thick radiator with 25mm fans is 55mm so there's still some 12mm left over.
In some mATX motherboards (like the B550 Aorus Pro), the full x16 slot is in the 2nd slot which lowers the GPU location. I could see this proving more difficult in a 4-slot case with a full watercooling loop (unless you go extra slim with rad and fans). I still see pros and cons with both 4 and 5 slots. Wouldn't yet know of cooling benefits I can try right now. Either way, with mATX you're gonna top out at one 240mm + 140mm.