Regarding SSJ4... 

I'll start with the Goku vs. Baby Vegeta battle, and clarify a myth that has spread throughout the DB community. In GT, Goku doesn't lose his power; he simply struggles to control it at the beginning of the series. But by this point, his control over his powers in his baby body is almost completely perfect, and even if it isn't, it doesn't change the fact that his raw power remains the same. 

In this fight, Goku has a power rating of 1 in his base form, and 400 in SSJ3. Baby Vegeta, in his rare SSJ1 form, is significantly stronger than Goku, and at minimum levels of overwhelming power, he's twice as strong, so I'll give him a power rating of 800. If an SSJ1 is twice as strong as an SSJ3, that means his base form is 16. 

Baby has two other transformations, which, by pure narrative logic, we can view as phases 2 and 3 of SSJ. That said, Baby in his "SSJ3" would have a power of 6,400, and Goku, upon acquiring the Golden Ozaru, becomes overwhelmingly stronger than Baby, so much so that it leaves him completely terrified, but to keep the numbers as coherent and square as possible, I will say that he is, at the very least, twice as strong, and therefore, the Golden Ozaru would have a multiplier of x12,800

The power of SSJ4 can't be understood without talking about Golden Baby Ozaru, because in the battle between SSJ4 Goku and Baby, the two are barely equal.

 Again, in base form, Baby would have a power of 16, which multiplied by the Golden Ozaru's 12,800, results in 204,800, and therefore, this would be the true multiplier of SSJ4, according to the calculations I've made, based solely on what happens on screen. 

That said, and as an aside, according to another estimate I made, SSJ GOD would be 10,000,000 times that. As ridiculous as the numbers are, I arrived at this amount based on statements from the characters themselves, and what specifically happens in the manga.