Chuunibyou: Quoting wikipedia, "Chunibyo (中二病 Chūnibyō) is a Japanese colloquial term that translates to "middle-school second-year syndrome" or "eighth-grader syndrome", typically used to describe early teens who have delusions of grandeur, that desperately want to stand out that they have convinced themselves they have hidden knowledge or secret powers." Though it is not limited middle school age.
4℃ is a chuunibyou, and Faris and Okabe at least act like they are true "chuuni"s. You'll see it in many other series as well, like "Love, Chunibyo & Other Delusions".
Kurisu will often say to Okabe when he gets too carried away, "chuunibyou ga otsu", which is an @channel slang to sarcastically say he's going overboard by saying he's working hard at it, if I were to roughly translate it.
4℃ (meaning 4 degrees Celsius), can be pronounced in Japanese as "Shido"; "shi" is one way to pronounce the number 4 and "do" means "degree" in this usage.
To go into it a bit deeper, Shido likes to say he "burns like dry ice", even though that is way colder than 4℃, but his name can still read as a "chuunibyou" reference to his being a "fallen angel". 4 is pronounced as either "yo(n)" or "shi" in Japanese. "Shi" is also the pronunciation for "death". "Dou" which is close to "do", can also mean "way", so "Shido" can sound like a play on words meaning "way of death".
Okabe likes to irk him by calling him "yondo" instead, and I think most Japanese would say "yondo" when talking about the temperature, since the similarity with the word "death" means they tend to not use that reading in many situations.
Even in the visual novels, he and his cronies, the Viral Attackers, remain two-bit antagonists, and yes, they are the thugs who chased after Faris and Okabe. In some of the spin-off VN's, they pop-up again doing various things like no good delinquents but not SERN-horrific level.