Broly is shown to have a tail as a baby.
But the tail is not there when he is an adult or kid, which means that he has the potential to become Super Saiyan 4. The only time he has a tail is when he was cloned, as shown in Bio-Broly. The time span in which Broly does not have a tail is: Broly — The Legendary Super Saiyan and Broly — Second Coming.
Do we know if Broly was able to transform into Super Saiyan 4 in Dragon Ball Z?
The only time he turned to Super Saiyan 4 is in Dragon Ball Heroes, which is a game and not canon.