First, they might be referring to a specific software or tool used in air traffic control, which isn't found on their system. The phrase "4 application not found top" is a bit confusing. Maybe it's a misstatement of "Application not found" error message when trying to launch a program. The "4" could be a mistaken key press or part of an app name they tried to access. Alternatively, could it be a specific ATC software they're trying to run? For example, maybe they are trying to install or run a simulation or specific ATC app that's missing or not compatible.