Sorry, my mistake: Ctrl+C copies, Ctrl+V pastes.
They are generic shortcuts for copy and paste.
On Discourse (this forum) works fine.
Same for right-click copy and paste.
Easier than save file + upload.
To answer the actual question here though:
Black playing at j4 is a big endgame move so typically important to play. But “at all costs”? Never! If black has something that will gain more than the ~7 points letting white play j4 will cost then do that.
It’s just that in many endgame scenarios nothing else will be worth losing 7 points in gote.