When a belt breaks immediately after installing it, this can ONLY mean one of two things. The first is that the belt was put back on incorrectly, and it may be pinched underneath of the beater bar. This would cause it to break immediately, but I seriously doubt after 4 belts that you put the belt on wrong all 4 times.
The second, and more probable cause is that your roller brush has gone bad. This could be why the belt broke in the first place. In order to test to make sure the brush is bad, unscrew the bottom plate, and remove the brush like you were changing the belt.
Hold the brush at the very end on both sides, using only your finger tips. While holding just the bearing caps, try spinning the bar with your thumbs. Does the brush spin freely in your hands? It should spin completely around 4-5 times on each spin. If the brush will not spin, or if you feel any resistance to it spinning, this is the reason why your belt continues to break.
If the brush can not spin, the motor shaft will continue to try and spin the belt, but since the brush doesn't spin, and the belt doesn't move, the motor shaft will just burn right through the back end of the belt.
If your machine is actually the model that's listed on this Fixya repair page, the Dirt Devil Featherlite model M085845, then your machine uses the following roller brush: Dirt Devil Plastic Brush Roll for Model 085845 Part # 2-881020-000
The next time that change the belt, install the brush back into the machine, but this time pull up on the belt towards the brush to make sure that the belt spins freely, and spins the brush as well. If the belt doesn't spin the brush, there's some issue, and the belt will break as soon as you turn the machine on.
I hope this helped you. If you find out that you don't need a new brush, feel free to use the comment section here on the repair page and let me know, I'd be happy to walk you through some more advanced diagnosing. If you found this helpful, please be so kind and rate my repair and / or leave feedback for me.
Thanks so much,