Installing heatsinks is not well documented. This youtube video shows how to install the heatsinks provided from Canakit, a North-American Pi supplier.
This case from c4labs is an excellent layered case with a built-in interior fan which is acceptably loud (price $25).
With heatsinks installed and the case fan operating, a Pi 4 reports an operating temperature of around 38’C.
This is another case from c4labs, this time it allows up to 8 Pi 4 boards to be installed in a cluster case.
In the picture below you can see 4 Pi4 boards, each with a Texas PoE HAT installed. Each board is plugged into a PoE port on a YuanLey switch and has a Kingston A400 drive attached.
Operating temperatures, with the Texas PoE HATs installed are in the range of 44-49C.
You may measure the on-board temperature for your Pi 4 by issuing the following command:
vcgencmd measure_temp