If Graves attempts to stack True Grit on any non-minion target (champions, monsters, etc.), it will not cap. For example, I tested versus Baron and reached 60 stacks, resulting in around 1000 armor and magic resist, as well as on an enemy champion, getting to around 10 before calling it bugged.
Steps to Reproduce:
Level up E
Auto attack to reduce cooldown of E and refresh True Grit duration
Repeat until stacks are above the cap of 4