As far as I'm aware, TrackIR does not enumerate as a joystick, so the setup you're suggesting consists of only four joystick devices.
I don't have any external USB hubs, so I can't do a practical test either.
This post suggests that even the simplest joystick interfacing method(predating DirectInput/DirectX) supports 16 devices.
Note that there may still be an application-specific limit, meaning the flight sim may not support as many joysticks as is theoretically possible.
This post says 8 is the maximum for CLoD.