Here is the trick.
Build up 5000 loyalty on one of your colonies. Kill, yes *kill* the ones the plan to cluster it with. Cluster. You now have a 5000 loyalty cluster made up of the 3 dead colonies and the one with population on.
This works since cluster loyalty is calculated by dividing the total loyalty of the planets to be clustered by the amount of planets to be clustered with population on.
This works for Guardian too, but you have to find a 2500 loyalty planet / cluster by fighting which is pretty hard...