4.5 but it isn't a huge improvement. The most significant addition is the small memory gain you get which is crucial given the limited memory the 8830 already has.
I hate OS 4.5. It adds the voice notes feature, the ringtones are louder and more apps are supported but the music volume is lower, and after playing one song it says "Device media processor is busy" and I need to take out the battery and reinsert it. I used it for a week, I didn't like it, I downgraded to 4.2.
