If the rogue planet is truly all alone in space, you're definitely right. 4 million times is a lot, but space is really, really big, and solar radiation falls off with 1/r^2.

Let's assume the auroras are proportional to the size of the magnetic field. That's probably not true, it's probably actually proportional to the square root of the magnetic field because field strengths fall off with 1/r^2, but let's give it the best possible chance of having huge auroras. That would mean that a planet with 4x the magnetic field of Earth would have the same Aurora brightness at 2x the distance. So, something with 4 million times the magnetic field would have the same brightness at sqrt(4,000,000) the earth-to-sun distance, or 2000 the distance. If it were in our solar system, or even just near our solar system, it would be bright. But, space is big.

Since the earth is about 500 light-seconds from the sun, 2000 earth-distances is about 1 million light seconds, or about 11.5 days. By comparison, the closest star to Sol is Proxima Centauri at 4 light years. So, these Auroras would only be earth-like if the rogue planet were very close to some star. It wouldn't have to necessarily be in orbit of that star, but it would have to be pretty close. If it were out in the space between the stars, there's just nothing there for the magnetic field to interact with.