With a PoD in 1900, this isn't that hard at all. There are plenty of ways to do it:
1. No WWI: This is by far the easiest way to achieve a multipolar world. The US would be the richest, sure, but it would only be "first among equals", especially with an industrialized Russia, Japan, Germany, CANZUK (Imperial Federation?), Ottoman Empire, France, and possibly India to compete with. Of course, China would be counted too, although its economic growth is likely to be stunted in a world where the Great Powers are free to carve it up as they did pre-WWI.
2. Early 20th century China gets its shit together: Depending on how everything goes down, this could easily lead to at least four powers (US, China, Russia, UK), if not more.
3. No Mao & No License Raj after WWIII: This gives you much faster economic development in China and India. By the late 80s, they're on par economically with the US and USSR, giving you at least four powers. This could go up to 6 or 7 assuming that Japan still does alright and some kind of smaller EU forms (Inner Six only).
Edit: And of course, with a little luck and economic development, there are plenty of other countries that could become major powers (Brazil, Indonesia, Mexico).