Why didn't Four know Tris?

Yes, in the books, Four is 18 to Tris' 16. That would mean, in a normal world school system, Tris and Four are just two grades apart.

And given the class size for Tris' Dauntless class (book 20, 10 made it)

Initiation is taken very seriously in Dauntless. When Tris went through Dauntless initiation, there were only twenty new initiates that made it to the compound after the Choosing Ceremony and only ten were accepted. The rest became factionless. The initiates are ranked together (Dauntless-born and faction transfers together) throughout the process and the higher the rank, the better job options there are. -Divergent Wikia

and movie (33 on the scoreboard, 24 made it), let's round up some to guess the size of the Choosing Ceremony.

If we assume 40 chose Dauntless (Transfer or not), and assume that each faction has an equal number of 16 year olds, and assume that the population has no growth or decline, so an equal number of kids every year (A lot of assumptions), we get 240 kids aged 10-16 in any given year in each Faction. With the 2 year age difference, that's 160 kids Tris' general age of which Four could be. That's a very small school system.

But this is Abnegation we are talking about. Very conservative, the type to eat small meals with family and very stiff upbringings that would not lead to typical high school behavior and fraternizing. I don't see any bond fires, high school sports, prom or similar things happening. I don't see Abnegation families inviting friends or co-workers over for dinner every other week. Tris and Four would, statistically speaking, and taking the faction behavior into account, just would never personally meet. Compare what we see of Abnegation to what Tris saw during the Initiation. Large mess halls, shared Barracks.

Movie is much easier to explain. Tris is 16~18, Four is 24. This was done because:

“Theo James who plays the love interest in my film is 28, in the book his character is 18, but in the movie we’re making him about 24/25. He’s kind of ageless in a way. And even though in the book Tris is about 16, we never allude to the fact that she’s that young.” -Shailene Woodley

That would, even in a very small school community, exclude any possibility of them knowing each other.

_{(Just a nice tidbit, old Dauntless are made to take a walk, Judge Dredd style. They can choose to die or choose to become Faction-less when they get older. Hence no old people in Dauntless).}

Actually, Tris mentions in Insurgent, that, while she never talked to Tobias in Abnegation, she did see him hiding behind a curtain at his mother's "funeral."

In Allegiant, Tobias recalls that, while it is true he never talked to Tris, he did notice her every now and then walking in the school hallways and down the sidewalks in the Abnegation sector.

In Four: a Divergent Collection, it is noted that colleagues did meet every now and then for dinner and bring their families. However, Marcus kept Tobias isolated, only letting him out of the house for school and important community events. Otherwise (such as in the case of being invited to dinner at his father's colleague's house) Tobias was forced to pretend he was sick. Whenever guests came to dinner at the Eaton house, Marcus had Tobias eat dinner early and then stay upstairs for the duration of the visitor's time at their house.

Four chose Dauntless and was very intelligent. As four went to dauntless that 2 year difference is a lot. Looks change too.