Quote Originally Posted by Tevin View Post
Proficiency should be +3 at 7 HD, no? That's how its attacks and saves are calculated currently.

How much of a boost to AC or HP would you suggest to boost it to CR 4?
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.