I knew about bent four since decades. I believe I have really understood it only recently. (But who knows, maybe that moment is still in the future.) It didn’t happen in my games very often yet though.
We have a thread for sharing sekis already, so why not one for bent fours?
I’ll make a start: here is one from a recent game of mine involving around 45 points in the lower right corner.
I have a thread where I work through how bent-four works under Chinese rules. Basically, the bent-four is dead in ko, which means that if there are big enough, unremovable ko threats, it could actually live.
Another example is in the last post of that thread:
Under Japanese rules, if one player has two bent-four formations in separate areas, then they would (most likely, unless some other weird connected things are happening) both be dead groups. The life and death resolution phase of Japanese rules applies special ko rules that would essentially isolate each position and nullify any ko threats. See:
Two bent-fours in opposite colors could actually result in a seki (under both Japanese and Chinese rules):
Unfortunately, the website that I linked there seems to now be down, but basically, it showed how the position is a hanezeki-like oddity, where it behaves like a strange capturing race, with the person who starts it would lose.
What is best play for black/white playing first under chinese rules?
(I have no idea about japanese rules, maybe @yebellz can analyze that for us!)
Although the position is presented on a compact small board here, it does not in fact rely on the two groups being adjacent (at least in chinese rules). This sort of situation could happen in a normal 19x19 game, but I don’t know of any examples from real games.
Under Chinese rules, both players should pass, and all stones are alive in seki.
Under Japanese rules, both players should pass, but all White stones are dead (and removed), and all Black stones are alive*.
*Even though it seems redundant, I’m pedantically saying that all of the Black stones are alive and that the dead White stones are removed, just to clarify that there are not any other weird situations going on, like dead stones not being removed due to anti-seki or residing in the eye of a seki.