The HP ProDesk 400 G4 SFF has one PCIe slot and tight cooler clearance

The ProDesk 400 G4 SFF keeps expansion minimal — there’s a single PCIe x1 slot for adding a Wi-Fi card, extra USB ports, or whatever else you need. It’s not much, but it’s enough for the basics.

If you want to swap the CPU cooler, you can, but you’re limited to low-profile coolers because the chassis doesn’t have much headroom. Standard coolers won’t fit, so measure before you buy.