I would have to say yes. Angels are territorial. The two you have seem to get along. Any changes to that may result in fights. I would assume that the newcomers would be the ones attacked. I think your tank seems fine as it is.
Two paired angels are all you need. However, the tetras won't stop the angels from laying eggs, they will predate upon the young. If you want to spawn your angels, they will need a dedicated tank of their own.