Chapter 151 of Dragon Ball actually proves the difference between Vegeta and Goku when it comes to transforming into Super Saiyan 4 in Dragon Ball GT.
How Dragon Ball, DBZ, and GT Take Advantage of Time Skips
Why does Super Saiyan 4 make GT Goku taller, and when Vegeta goes super Saiyan 4, he's the same height? - Quora
Why have we not seen Vegeta and Goku go Super Saiyan 4? - Quora
Dragon Ball Z: Kakarot Needs GT DLC
Super Saiyan 4, Dragon Ball Wiki
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A Dragon Ball Beginner's Guide: Where To Start Watching The Classic Anime
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Dragon ball GT Goku Dragon ball, Goku, Dragon ball super goku
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Every Important Dragon Ball Transformation, Power-Up, and Fusion So Far - IGN
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Dragon Ball: Every Super Saiyan Form, Explained
Dragon Ball GT: 10 Things Everyone Forgets About Super Saiyan 4