As previous posters also has stated there is no direct correlation proven between brain size and intelligence. A possible venue of investigation would be brain wrinkles or folds. (Gyri and sulci) These create a greater surface area in the brain and may be linked to intelligence and greater processing capacity.
Please note that the dolphins brain (left) is roughly the same size or and in possession of more gyri and sulci than the human brain (right) and yet the dolphins have not invented fire, thermonuclear war or market economy, but instead just swimmed around eating fish and making wicked back flips. So the case for intelligence may be circumstantial.
An octopus brain looks completely different from a mammal brain. It is more or less completely alien to us. They seem to have a distributed brain with the majority of their neurons actually residing in their arms (which have some semi-autonomous control over movement and their own sense of taste) and a lot of it's processing capacity seems to go towards instinctively controlling a highly complex body.
So what is required for your tetrapi to be smart? Well... actually most likely not much. They are probably already smart (as I recall there is a Question regarding this already on the site, you should check it out if you can find it) and what they lack is social structure (cephalopods being known for being solitary, aggressive, and canibalistic by nature, although there is evidence for them to be surprisingly social as well... so you probably need to talk to an cephalopod specialist, not me). From what I have understood, though, the main case against cephalopod civilization is not their brains nor their smarts but rather physical limitations in terms of life span and gestation. They spawn their young in a fire and forget way and never care for the offspring, which puts limits on social development. Also they die after spawning, making it hard to become a venerable elder and carry on the legends of your people.
Solve those things (moving from water to air will probably do that actually as hatching a litter of a thousand might not be a viable strategy compared to carrying the young internally) and you'll find there actually may be no limit to your cephalopods.