The clearest indication that Broly's powers are tied to the Super Saiyan 4 form in "Dragon Ball Super: Broly" is that his final powered-up form in the movie has him sporting golden pupils. Fans who watch "Dragon Ball GT" or have some experience with the other non-canon outings of the franchise will recognize immediately that this characteristic is unique to that form.
However, the fact that we still haven't seen the emergence of Super Saiyan 4 in the five years that have elapsed since "Dragon Ball Super: Broly" was released suggests that the creative team behind the series may be hesitant to bring back the form. Furthermore, the emergence of seemingly more powerful variants like Super Saiyan Blue and Ultra Instinct could suggest that the form has been rendered superfluous.
Even with this in mind, it's still possible that "Dragon Ball Super" could still bring back the form, if only as an exclusive transformation designated specifically for Broly. This would allow for the best of both worlds, where the form could return for fans of the design and mythology behind it without mucking up the continuity of the current series.
