i'm planning to buy an Raspberry Pi 4 Model B. Short time after release of the Pi 4 many people have complained about heat problems (not only at 100% CPU usage, but in idle). My question is now, did the firmware updates from the past fix these heat problems especially the ones which happened in idle?
Don't expect there to be a firmware fix for the RPi 4 heat dissipation. That's the way the chip is built: if you want more performance, you need more current and there will be more heat. All single-board computers similar to RPi 4 in terms of performace (Orange Pi PC, Odroid, etc.) need heat sinks or cooler fans, and will go into thermal throttling without cooling.
Practically, RPi 4 is only usable without additional cooling if it sits idle most of the time. Even then, a passive heat sink on the SoC is recommended: the SoC will be substantially cooler and will be able to perform typical tasks like an upgrade without slowing down to a crawl. However, if you plan to load more than 1 CPU core for long periods of time, you will have to install active cooling, i.e. a heat sink and a fan.