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Grade 4 is essentially just Super Saiyan buy with better control, allowing them to use more power more efficiently. It shouldn't really be considered a different form.

Also, it was implied in the Boo arc that none of the Saiyans are in this state anymore. When Goku suggested that he and Gohan be Super Saiyans when fusing with the Potara, Old Kai advised against this, saying being in that form would cause too much stress on them and shorten their lifespan.

For them to be in Grade 4, they must always be transformed. The main reason is because they use up too much energy just initiating the transformation. But after the Cell arc, they no longer maintained the form and just say in their normal form during everyday life. This implies that they are no longer using Grade 4 when they transform. Because they no longer maintain that form 24/7. Either that or because they have mastered Super Saiyan to such a degree, many of the drawbacks are not as extreme as before.