When I read that, I actually got the impression that the towers are just entrances to a single, much larger facility... similar to how the shrine entrances are pretty small compared to the actual shrines underneath.
I'm not sure if the fifth dungeon will actually be a divine beast... there is a mention of there being some sort of ancient facility beneath Hyrule Castle, so I think it'll be that.
If there was a fifth divine beast, then I think it would have been mentioned at some point, but it's possible they...