Apple is no longer signing any version of iOS 8. You can't get rid of iOS 9 on your iPhone. Are you a registered Apple developer? iOS 9.1 beta 4 was just released to developers earlier this week.
If it says there is an available software update it means that it has not actually been installed yet. To find out what iOS version you're currently on, go to Settings>General>About and scroll down the list till you get to version.
If you're not a developer, or didn't join Apple's developer program, you've signed up to public beta.
9 is the current software version. In your case, it's 9.1, which will be released to the public some time later.
Beta means it's not public, it's a software that's tested, it's pre-release software. 4 is the 4th version of the 9.1 beta software.