One idea could be Pan discovers a hidden power within herself that allows her to transform into a unique form related to Super Saiyan 4. Maybe she has to go on a quest to find the knowledge to fully control this power. Another plot could involve Pan traveling to a different dimension where Super Saiyan 4 is the norm and she has to prove herself among stronger Saiyans there.
A possible plot is that an old enemy returns and only a Super Saiyan 4 - level power can defeat it. Pan, being the young and determined Saiyan she is, tries to achieve this form through intense training with the help of her family. She could also face some internal struggles, like doubting her own abilities, but overcomes them as she gets closer to attaining the Super Saiyan 4 state.