I am selling my P4 and was wondering how you go about resetting back to factory settings, or do you just sell it as it is ?

You could do factory reset through the Assistant 2. Please download it on DJI official website: http://www.dji.com/phantom-4/info#downloads
Then install it, and connect your aircraft to PC or MAC, you will find the option.

Thank you Natalia

I have also read that it can be sold without doing a reset and the only personal data stored would be the last few flights, is that correct ? just be nice for the new owner to be able to fly straight away once he has linked the P4 to his DJI go app is that correct ?