Hello

[spoiler]
4th Pyramid is located in Archives on Bloodmoon Island.[/spoiler]

Be aware of fact that Pyramid CAN BE DESTROYED by AoE skills and such while it's dropped on the ground. I had yellow pyramid dropped on the ground near source fountain in the basemenet so I can fast tp-in to refill my source points.

Then there was some fight in the basement and my pyramid got destroyed once and for all :( If you want to have fast tp to source refill place, put it into some cabin there.