As a newbie I had some trouble sorting this, so I am setting out the steps that FlightAware and others on this forum showed me. Basically the PiAware SD image will not work on the Pi 4, as it needs updating. However, PiAware can be installed successfully using the command line in Raspbian Buster.
You need to type the following lines. Each needs to be loaded with the Enter key:
sudo apt update
sudo apt dist-upgrade
sudo dpkg -i piaware-repository_3.7.1_all.deb
sudo apt-get update
sudo apt-get install piaware
sudo piaware-config allow-auto-updates yes
sudo piaware-config allow-manual-updates yes
sudo apt-get install dump1090-fa
After all that, make sure you claim on the FlightAware site and enter your location co-ordinates. If you don’t, MLAT will not work!