Windbreaker 216 made me so crazy I decided to stop lurking. In my frantic post-chapter frenzy I ended up opening several academic articles, so there might be wilder posts to come, but first I wanted to touch upon Suo's connection to the number 4 and what 4 signifies in Chinese culture.
A lot of people have brought up the four = death connection, which is correct, but I feel like it's important to mention that the number 4 is a bit more complicated than that. Four is a bit of a weird number, because it's both lucky and unlucky simultaneously.
For example, there are a lot of good things that are grouped into four. There's the Four Auspicious Beasts, which are associated with the four cardinal directions. There's the Four Gentlemen, which are four plants representing traditional virtues and the four seasons. There are the Four Great Beauties, the Four Great Classical Novels, etc.
Actually, while googling to fact check myself, I discovered that there are even some folk wedding traditions involving the number four. There's the concept of "四红四喜", a wedding tradition where the bride wears four reds that symbolize four happinesses, and the four fruits, which are used in the wedding ceremony.
However, tetraphobia (fear of the number four) is still definitely real. You're not supposed to get married on a day with 4 in the date, and buildings will avoid having floor numbers with 4 in it.
I find this concept kind of intriguing when it comes to Suo. Four is connected to death, but also to the four happinesses, which include longevity (arguably the opposite of death). Suo's name references resurrection, a transition between death and life. Four is both lucky and unlucky, and in a way that makes it a sort of mysterious number, and isn't that kind of fitting for a very mysterious guy...
Anyways, I don't have an exact conclusion to this. I feel like four = death is a pretty well known thing, but it's a bit more complicated than that and I wanted to mention it. And something about the whole "stuck between two meanings" thing is kind of making me think of being stuck between two organizations? Maybe someone else will think of other directions to take this, I believe @silfurskin mentioned that four also has organized crime connections.