The question of which of Goku's power-ups is stronger, Ultra Instinct Goku or Super Saiyan 4, has finally been answered in the Super Dragon Ball Heroes manga. The promotional manga is for the digital card game that sports the same name, but it is not regarded as canon to the events of the Dragon Ball franchise. Still, the series has found a way to pit SS4 against Ultra Instinct to see which of Goku's forms is better.
https://screenrant.com/dragon-ball-ultra-instinct-vs-super-saiyan-4-settled/