I read that the official heasink is not actually enough for heavy compilation tasks. Are there bigger heatsinks compatible with Rock Pi 4?
What kind of heatsink do you recommend for heavy compilation tasks?
Use a fan with it.
I am not going to use a fan.
I would strongly advise small heatsink with Noctua A4x10, like on photo
But if you insist on passive cooling - that’s gonna require a bit of handmade. You will need something like this
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According to https://youtu.be/l6EUfqywsOI?t=809, with official large heatsink, temperature reaches 66.1’C after 8 minutes of heavy CPU usage.
That’s within the limits. 
It only starts to throttle near the 75 C mark from my experience.
I have a Rock Pi 4 v1.3 with the big heat sink. So far I didnt face any issue regarding temps. Have run a kernel compile (~ 1hr) as well as software compile (mesa, 1/2 hr) without issue.
Only precaution with big heatsink is while installing: to ensure the raised sides around the CPU are properly cut off.
What is the CPU temperature while you compile kernel?
Tested today. Ambient temperature 24 C.
In the beginning (after some web browsing): 44 C
5 min in kernel compile: 61 C
15 min in kernel compile: 76 C
30 min in kernel compile: 81 C
40 min in compile: 82 C
All cores were in use. Big cores go upto 2 Ghz, small cores upto 1.5 Ghz (slight overclock).
Buy yourself a 4 foot by 4 foot by 1/2 inch sheet of aluminum.
Buy “official” heatsink.
Grind paint off heatsink.
Drill holes through aluminum sheet and “official” heatsink. Tap the holes in the heatsink, and bolt together with some thermal paste between them.
That should be sufficient to keep the thing cool indefinitely without a fan at ambient temperature up to (but not including) baking in direct equatorial sun.
Now what I really would suggest as a better option, is that you use the crappy rk3399 for something that is not all that demanding, like playing movies on your TV, and use something stronger for compiling kernels, like a recent x86 desktop.
I used to do that and realized that faster compilation is not worth the extra maintenance burden.
It may consume less computer time but more human time.
I want to minimize human maintenance time.
Kernel cross-compilation may not be that time-consuming, but try compiling every ARM64 package on an AMD64 machine(I use gentoo). You are going to have to spend a lot of time on troubleshooting because many distro packages fail during cross-compilation.
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If I added an M.2 NVMe SSD and two USB 3 devices to Rock Pi 4A, would large heatsink be still sufficient? I guess so because thermal throttling kicks in, but if thermal throttling is not enough, things may get ugly.
I want to add a USB 3 gigabit ethernet adaptor and a USB 3 802.11ax adaptor to Rock Pi 4A so that it becomes a wireless router.
The NVMe SSD can go under the big heatsink or over the top, so should not interfere with the cooling of the Rock Pi. Thermal pad or Heat sink is available for the SSD too if needed.
I cant say how much the USB 3 gigabit ethernet adapter or USB 3 802.11ax adapter would heat up, thermal pads or small heat sinks could be used there too.
If thermal dissipation is needed, can mount it on the side or use a small quiet fan like the one suggested by Dante earlier. I would recommend the Rock Pi acrylic case too, it adds structural integrity to the whole setup.
This thread has some nice info regarding cooling solutions and cases:
The problem is heat from USB 3 controller and NVMe controller transferred to CPU. Raspberry Pi 4 had more heat problem when USB 3 controller was used.
Using the Usb 3 ports, i didn’t face heat issue. The temperature results posted earlier were running Slarm64 from a USB HDD.
Dont have an NVMe drive so cant say about that.