I would like to know if the warranty of my phantom 4 pro will be maintained if the aircraft crashes when using Litchi.

Only if aircraft has a malfunction which is not caused by litchi app .

You will have no problem IF you can show that the problem was caused by DJI's equipment or programming.
The catch is that this might be difficult with Litchi, particularly if you have lost the Phantom.
The internal flight recorder will show DJI engineers what happened and what caused the problem.
But if you have lost the Phantom it might be very difficult to provide the necessary proof as Litchi doesn't create a flight log showing all the things that DJI will want to investigate.