)Note: this is just a fanmade version based on the original Last Breath by ZerJox)

Last Breath Phase 4 is said to be the result of Error attempting to manipulate and possess Last Breath Phase 3 from Gaster. However, he failed and accidentally caused Last Breath Phase 3 to progress to Phase 4 leading to a fight that broke out, but since Sans is already dead, he is only temporarily maintaining immortality and when time is , he will die.

Error had a hard time resisting a stronger version of the regular Last Breath variant but in the end, he succeeded in capturing the soul of Last Breath Phase 4 and keeping him alive. They are all just puppets of Error. Perhaps this is a great success of Error.

(You continue the rest, have fun)