Steps to reproduce:
1. Opponent has Tellah, Great Sage on the battlefield
2. Opponent casts a 4 mana spell
3. While Tellah's ability is on the stack, cast syncopate on the 4 mana spell with x >= 4
4. Opponent pays the syncopate cost
5. When Tellah's ability resolves it will recognize 8+ being paid instead of just the 4 that was used to cast the spell.

I have this confirmed in my 17lands logs here (only the last turn matters):
https://www.17lands.com/history/e7c38eb0fb9b44288f76c2e4c604994a/8/0