How much temperature is normal temperature for Raspberry Pi 4

What temperature range is normal or... how hot is too hot?

My Pi 4B's with a decent heat management system (I've used several varieties) will typically run in the mid to upper 30°C's to the low-mid 40°C's when either idle or under typical/normal workloads (not pegging the CPU cores).

Anything under 50°C is great!

It is possible to stay in this temperature range without fans (more on that shortly).

The Raspberry Pi does have some self-protection against over-heating. Specifically it will reduce its clock speed so it can run cooler. The drawback is that it's not doing computations as quickly.

The throttling temperature is controlled via firmware and will throttle at 80°C (But a firmware update could change that. So this value may not be the same in the future.)

Fan Noise

You can think of Raspberry Pi cooling in a few ways and... whether or not the Pi is in a case, the case type and venting, will also impact these.

1. You could run a stock Raspberry Pi with nothing added to help with cooling.
2. You could use a passive heat management strategy involving a heat-sink, but no active cooling system (no fans, no water pump, etc.)
3. You could use an active cooling system such as a fan, but but no heat sink.
4. You could use both a heat-sink system and an active cooling fan.

There are lots of variations on fans. Many are always-on and at just one speed. There are fan solutions that only kick on when the temperature reaches some threshold. There are also multi-speed fans.

There is also a variety of quality among fans. I've been disappointed with several fans that initially seem to be fine... only to start hearing excessive fan noise a few days later (the bearings are already have problems).

Noctua brand is known for their high quality and they are one of the least noisy fans. They do cost a bit more. A cheap fan might be in the range of $4-8 US Dollars. A similarly sized Noctua brand fan would probably be more like $14. Be aware that they make these in 12v and 5v versions and you want the 5v version for a Raspberry Pi and likely the version that does not use the PWM (Pulse Wave Modulation -- which they use to control speed so this is a variable speed fan. But Noctua expects a 5v PWM and I think the Pi uses a 3.3v PWM and I'm not sure that can reliably control the fan without problems. So I chose to get the basic single-speed (non-PWM) 5v fan. This brand is unlikely to be excessively noisy nor start to make the buzzing sounds of a failed bearings (at least not any time soon).

Passive Cooling

Not all passive cooling systems are equal. The basic idea is to install a tiny heat-sink (fins) using some thermal tape. These provide only moderate cooling help and usually don't do an adequate job without also adding a fan.

Recently (a few weeks ago), I purchased an Argon One Pi 4B case for my Pi (Argon40.com). This is an interesting case because it also has a small board that attaches to the side of the Pi and routes ALL connectors to the rear (power, HDMI, and audio are all routed to the rear next to the USB and Ethernet ports) and also supplies a power switch. The case includes a variable speed fan and software to determine when to activate the fan and the power level. Their 'default' values are no fan below 55°C. At 55°C they activate 10% fan. At 60°C they boost to 55% fan. At 65°C they go all the way to 100% fan.

But the entire upper case body is an aluminum alloy casting and they have solid posts that hang down from above and touch the CPU (System on a Chip) via supplied thermal tape. When I put my Pi 4B in that case... the temperatures (the Pi was previously in a case with basic heat-sinks and a fan) that were previously in the mid-40's... dropped to the upper 30's. When I apply typical workloads I sometimes see it go into the low 40's.

If I apply synthetic loads to send all 4 cores to 100%... I have managed to get the Pi to about 46°C. I once managed to run it hard enough and long enough to finally see it hit 51°C. I have never managed to get it any hotter than this ... no matter how hard I try. Keep in mind... this case doesn't bother to turn on the fan until the temps reach 55°C. This means I have ... never once ... had the fan turn on in all these weeks of trying. I would say it has outstanding passive thermal management. (The case will feel warm).

This case is a bit on the pricy side ($25 is more than the cost of most Raspberry Pi 4 cases) but it has several nice features and, in my opinion, it's worth it. The same company Makes a more compact case uses the same passive thermal management (at $15 USD). That case does not have an option for any cooling fan. But when I noticed how exceptionally well their other case is able to dump the heat, I'm starting to think that the their cheaper case would have been more than adequate (I have not tested one, but I'm thinking I will because I own several Raspberry Pi 4B's.)

Full disclosure: This is testing in my home office, which is located in the basement of my home. Since hot air rises, the basement is naturally the coolest room in the house. It tends to be around 22-23°C (the upstairs is much warmer). So I suspect if I tested the Pi upstairs I might be able to get it warm enough to kick on the fan at the lowest speed.

I have looked at a few other passive thermal cases... where the entire case body (top and bottom) is all heat-sink. These cases are not expensive ($10-15 USD)

I would not use the "official" case for the Raspberry Pi 4B. The case is basically sealed with no way vent any warm air.