Why carl weathers act in rocky IV?

Carl Weathers, who played Apollo Creed in the Rocky franchise, did not act in Rocky IV.

Apollo Creed dies in Rocky IV, during his fight with Ivan Drago. This death sets up the main plot of Rocky IV, with Rocky training to avenge his friend's death against Drago in the USSR.

However, Carl Weathers does appear in Rocky V, which came out after Rocky IV.