it isn't clear in the manual whether to install the other 2 clips or if they are just spares!?

its only two! the other two are for spare parts in case one is broken or you need to remove it... the gave you two because if you need two remove the gimbal you have to break them...

Alright thanks! I figured as much. I've not installed them. But I wanted to check before flight.

Sheet I fitted all 4

Manual states to have them diagonal from each other (only two).

The metal bracket camera crash saver I bought came with two zip ties for the empty holes which I used. They don't affect the motion of the gimbal base so no jiggles but if the gimbal ever does break free in a crash, the zip ties will hold onto it so it does not go flying away. You can get two small zip ties and attach them loosely through the two empty holes.