When the Sweathogs are afraid Gabe is going to quit as their teacher, they come to his apartment to throw him a potluck dinner to convince him to stay.
When Arnold is promoted to mainstream classes, Gabe tells the rest of the Sweathogs to back off from spending time with him so he can flourish and make new friends in his new class. Arnold, however, is miserable and lonely without them and the Sweathogs feel the same way. They apologize to Arnold and tell him they need him even if he's smarter than they are.
When Epstein expresses his desire to become a vet and is told he's not smart enough, he gives Gabe and Julie all of his pets. When one of the hamsters turns out to be pregnant, though, Gabe calls Epstein in to handle the birth and tells him that he's got what it takes to follow his dream.
Julie's character arc in season 4 has her becoming a mentor for the Sweathogs, whom she previously mostly tolerated for Gabe's sake.
When Arnold joins a cult because he feels unwanted and unappreciated by the other Sweathogs, they spend the whole day and night trying to talk him out of it by admitting they go too far in their teasing, and how much they care about him.
Arnold and Mary's relationship is one big heartwarming moment from beginning to end. Highlights include Arnold talking her down from trying to kill herself, the two confessing their love for each other and Mary showing she's gained a lot of pluck and spirit by reassuring Arnold when he panics shortly before their wedding.