Seems like its not the LCD in my case.
I did a full strip down of the Hero 4 and could see no PCB damage from the saltwater. All the components were fully dry with no signs of residue.
My first attempt lead me as far as removing the back LCD housing as I was unsure how to remove the LCD from the housing. I couldn't find any vids, pics or anyone that had tried.
I did think I was at a dead end but a week later I took the plunge and bought a new LCD. Removing the LCD is very tricky as there is a metal clip locking the battery compartment in which has to be removed first before you can access the LCD. When you have freed the clip plus the two screws a hinge is also holding this down and runs along the bottom so you need to pry this too, this can be very tricky. Once all those are free the battery compartment can be removed straight up.
The LCD is stuck down by strong adhesive and took allot of force to push this through from the inside. As I did this I didn't realise the LCD on/off button is hinged in its slot and before I knew it the PCB cable snapped to this so be warned if you intend on using this part.
Replacing the new one was easy but this has not solved my problem as I am still unable to change the brightness so I think its something on the mainboard thats damaged. I'm not going replace the mainboard as I've spend to much on this already.
Hope this might help anyone else needing to remove this.