I think I know two of them, which are:
* Level 1-4
* Level 5-9

Is the next bracket 10-14? And does each bracket contain 4 levels?

Thanks in advance,
Potato

I think it's 1-4 4-7 5-8 8-12 9-13 or something like that.

so basically after level 4 you can't "twink", is that what you're saying?

Not sure what do you mean.

Sorry my bad for not explaining.

"Twinking" is an MMO term that refers to XP locking yourself at the top level of a PVP bracket. This means you're only up against either people of your own level or those below you.
Usually also means that your character is geared with the best in slot gear for that level, making him very powerful

Yep, you can't do that. All the way to 30 you will have to play with bigger levels sometimes.