Depends.

Does it come before or after? If it came after, there’s a high chance it would be more powerful easily. I mean think about it. People surpassing God forms especially mortals in Super was done quite easily. Either through invisible, off-screen buff or too quick upgrade in strength so I don’t see why Super Saiyan 4 which is another transformation can’t be treated the same.

Trunks out of nowhere got Super Saiyan Rage that boosted him to Blue Levels.

Gohan got Beast form purely through rage boost and is seen fighting on par with Ultra Instinct Goku.

So I don’t see why Super Saiyan 4 should be treated differently. It would work pretty much like these transformations but with better lore to it. Ikari form already is similar to it in ways and we all know how powerful Broly turned out to be, he’s very much like a Super Saiyan 4 with his adaptability and potential plus using Great Ape power with Super Saiyan. Super Saiyan 4 is highly adaptable as same techniques do not work again on it and Broly has shown similar adaptability.

Super Saiyan 4 also works like the Ultimate form, bringing out the dormant power of the user and we saw Ultimate Gohan with little training reach Blue Goku so it’s not hard to assume this form can surpass God especially with how Super treated the God form later.

My guess is very strong compared to God. The timing matters here. Maybe it even reaches or surpasses Ultra Instinct if it’s introduced now. However, if it’s somewhere after God, it would most definitely be more powerful and by a lot. If Goku and Vegeta awaken it, the best thing they could do is combine it with God Ki and Ultra Instinct/Ego.

The real Super Saiyan 5.

So there you have it. In my opinion, Broly already is a kind of Super Saiyan 4. He’s a true example.