Ok, Tirek questioned there being a 4th princess, but this makes no sense. He shouldn't have even known there was a third (cadence). When his brother tells of his plans, Luna is next to Celestia. Which means Tirek was banished over 1000 years ago. Cadence does not appear to be that old and it's revealed in the crystal heart book that she was born a pegasus. I don't know how old she was when she became a princess, but she doesn't seem old enough that Tirek would know about her. Yet, somehow he knew about her because he asked about a 4th princess but not a third. He does meet her, but shows no sigh of confusion by it. He simply trys to take her magic.
Tirek pointing at stained glass window S4E26

Questioning twilight as the 4th princess

Scorpan warns Celestia and Luna S4E25

Informing the Princesses of Tirek's plans