It's not my wiring mine would be far tidier and a small zip tie would hold the cables in place. Perhaps it's not the best photo to demonstrate telephone cable terminations. More cables terminated is due to that being an extension that's feeding another extension. 4 is indeed unused.Do phones still need the bell wire (other than retro phones)?
Why have you got two wires in 3 and 2? If you had two in 2,3 and 5 I would have assumed that you have an extension running from it but I can't see how that extension would work with only 3 and 2.
I vaguely recall being told that 4 was reserved for "future use". I never did find out what that future use was.