The manual says, "Mavic Mini has three flight modes, plus a fourth flight mode that the aircraft switches to in certain situations".  What is the "fourth flight mode" and what "situations"?  Reference: Mavic Mini Manual

I think when you lost GPS signal or fly without it (indoor) and use only vision systems.

It is called Attitude mode, this happens when drone has lost gps

If it loses GPS reception (because you're flying indoors or under a bridge or in a deep valley or between tall buildings, etc) then the Visual position sensor (VPS) will still function to hold the drone's position whenever your control sticks are centred.  However, if it's too dark or the surface is all one uniform colour with no detectable patterns, or the drone is flying too high (~10m) for the VPS to be working AND you lose GPS reception as well, then the drone will switch to ATTI mode.

It'll still fly and respond to stick inputs, but it will be unable to automatically brake to a stop and hold a fixed position when your sticks are centred.  Instead it will drift with the wind.

I did a little video a long time ago with my Phantom-1 showing this.  The controller had a switch allowing you to manually switch to ATTI mode whenever you wanted.  It was a nice feature, useful for getting smooth video because the drone would continue to drift along smoothly when the control sticks were released, rather than abruptly braking to a full stop.

Zbip57 Posted at 6-10 17:11
If it loses GPS reception (because you're flying indoors or under a bridge or in a deep valley or between tall buildings, etc) then the Visual position sensor (VPS) will still function to hold the drone's position whenever your control sticks are centred.  However, if it's too dark or the surface is all one uniform colour with no detectable patterns, or the drone is flying too high (~10m) for the VPS to be working AND you lose GPS reception as well, then the drone will switch to ATTI mode.

It'll still fly and respond to stick inputs, but it will be unable to automatically brake to a stop and hold a fixed position when your sticks are centred.  Instead it will drift with the wind.

Thanks for a complete and concise explanation along with a nice video.  I learned a few things!  

I think you will find that ATTI is when neither GSP nor the vision position sensors are available to the drone.
Where I am indoor flights have no GPS but I still have P, S and C modes available and the drone's hover is stable due, I presume, to its use of the vision sensors.
When indoors (no GPS) with the vision sensors covered by tape I see "attitude mode", with the vision sensors uncovered i.e. working, I think a poorly lit room will also trigger the attitude mode but I can not confirm that.

Hello there Wrongway. Thank you for reaching out and for sharing these information. In addition, the drone automatically changes to ATTI mode when the vision system is unavailable or disabled and when the GPS signal is weak or the compass experiences interference. When the Vision System is unavailable, the drone cannot position  itself or brake automatically, which increases the risk of potential flight hazards. In ATTI mode, the drone may be more easily affected by its surroundings. Environmental factors such as wind can result in horizontal shifting, which may present hazards, especially when flying in confined spaces. Thank you.

DJI Stephen Posted at 6-15 21:38
Hello there Wrongway. Thank you for reaching out and for sharing these information. In addition, the drone automatically changes to ATTI mode when the vision system is unavailable or disabled and when the GPS signal is weak or the compass experiences interference. When the Vision System is unavailable, the drone cannot position  itself or brake automatically, which increases the risk of potential flight hazards. In ATTI mode, the drone may be more easily affected by its surroundings. Environmental factors such as wind can result in horizontal shifting, which may present hazards, especially when flying in confined spaces. Thank you.

Thanks!  Adding this to my notes.  Good thing to know!  

Wrongway Posted at 6-16 19:44
Thanks!  Adding this to my notes.  Good thing to know!

Hi there Wrongway. You are very much welcome and thank you for the reply. If you have any other inquiries or concerns with DJI. Please feel free to post it here at DJI Forum. We are all here to help and support you.