5e uses CR to calculate proficiency bonus, and the number of hit dice is mostly irrelevant. See page 274 for the table listing what bonus a monster has at each total CR.
And using the DMG figures, a total of hit points to 82 (11d8+33) hit points would bring it to CR 4. That's about a 60% boost to how sturdy the monster is, and leaves is average damage per round untouched. Technically the RAW calculations work out that you could give it up to 105 (14d8+42) hit points and it would stay CR 4.