I started with a Raspberry 3B (not plus) as the 4B wasn’t available at this time.
It has only the passive heatsinks on it and is mounted into a cheap plastic case.
Pre-Covid times i was able to receive 3.000 aircraft over the day and never had any issues with heating, even not in summer where the temperature in my studio under the roof went up to almost 30°C.
The CPU load is over the day almost the same, independent of the traffic.
I moved then over to Raspberry 4B with my second device, but it is running more than only the feeders. Currently it runs the feeder, the WeeWX weather station and the monitoring of my personal cloud.
In reality the AirSquitter (Radarcape) is a small Orange PI with only 512 MB RAM and it works without issues also in a high frequency area. But it’s not using dump1090 but an optimized self developed decoder software from Jetvision.
Currently here in Germany a 4B with 2GB is cheaper as any other model except the Zero