/usr/bin/fastcgi-mono-server4 is in mono-fastcgi-server4 4.2-2.1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
exec /usr/bin/mono $MONO_OPTIONS "/usr/lib/mono/4.5/fastcgi-mono-server4.exe" "$@"
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