Was Broly able to transform into Super Saiyan 4 in Dragon Ball Z? 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.
Broly wasn't a canon character and Super Saiyan 4 isn't a canon transformation, so the idea of Broly being a canon Super Saiyan 4 is kind of madness.

Broly is a character from the movies, in which there is no hint that he or anyone else can become a Super Saiyan 4, and SS4 is from GT and there was no mention of Broly in GT. If he went SS4 in some game, then in the canon of that game he is able to, but otherwise I would say no, there is no reason to think Broly could be a SS4, as even his normal Super Saiyan form is in fact pretty weird and not particularly similar to the main characters' Super Saiyan transformations.