As of February 2020, the ABS case for Raspberry Pi 4B is upgraded with screwless mounting for cooling fan :)
The case comes in 3 parts, bottom, middle and top lid. The lid is being upgraded to enable screwless mounting of a cooling fan. Same as the assembly of the case itself, no screw is needed.
The case: https://www.cytron.io/p-bo-rpi4-b
Cooling fan: https://www.cytron.io/p-fan-0305-3008
Raspberry Pi 4 Model B 1GB: https://www.cytron.io/p-raspberry-pi-...
Raspberry Pi 4 Model B 2GB: https://www.cytron.io/p-raspberry-pi-...
Raspberry Pi 4 Model B 4GB: https://www.cytron.io/p-raspberry-pi-...