I have been testing a rpi4 and comparing it to the rpi3. The Rpi4 is actually the first Pi that is usable as a desktop computer, but have seen thermal issues like you. Many the early reviews have only done short tests with the cover off. The thermal CPU throttling on the RPI4 is huge, while the RPI3 might slow things down 50-30%. The RPI4 will slow down to 100%. In the case I tested the RPI4 was running close the RPI3 speeds. I don't think a heatsink or CPU fan is needed there just not enough heat being generated to warrant the expense.I have had good luck with running the RPI with just the cover off, and some sort of air circulation in the room.

I have been trying to figure out the cheapest way to cool the RPI4, I am thinking a single slow 5V fan blowing across the card would be the best solution. This would also cool off the VLI, and take up less room on the card. I am just hoping someone will make the case, fan combination the works at a good price point. If things get too pricey then other SBC look very interesting.

I am really disappointed the thermal issues were not considered. A simple fan header like on UDOO boards, or an official case with space and air flow so a heat-sink could actually work would make me feel a lot better.