How can I connect a Hailo-8 to a Raspberry Pi 4?

You will need a Raspberry Pi Compute Module 4 and a base board. The standard Raspberry Pi 4 uses the PCIe interface on board for a USB controller. The Compute Module 4 makes PCIe (1 lane) available on the module connector.

You can use the Raspberry Pi Compute Module 4 IO Board with a PCIe adapter.

Another option is the Waveshare Mini Base Board. It has a M.2 connector at the bottom.