I had very frequent GhostTouches as well and sent the device in for repair.
The screen has been replaced but no further information was given to me.
On the return shipping UPS had difficulties finding my adress and I was not able to update it online. UPS did also not react on a Mail. So they sent it back to the French repair center.
And that is where the story becomes really disappointing. I wrote during two weeks several E-Mails to the Fairphone support but became no Feedback at all. one week ago I decided to contact them by Phone except that the friendly lady ensured that they will take care now. Nothing happend. 
So I am now 4 weeks without the quite expensive Phone and am still not able to test their fix.
The good thing: I use a 6 years old Samsung Galaxy S7 as a replacement and except the small storage It’s not too bad
- No Ghost inputs
- Acceptable Battery life
- Good and quick camera
- no trouble connecting via bluetooth
- Good sound quality during phone calls
If the FP4 finds its way back to me I assume I’ll sell it even if the Idea of an sustainable product was very appealing to me 