What are the disadvantages of upgrading to iOS8.0

It means that you need to be running the most current iOS in order to run the app. The developer has upgraded the app to run on iOS 8 or higher, so you must be running that level of iOS in order to run the app.

Back up your device before upgrading, either to iCloud or iTunes, and then upgrade your device either OTA via General>Software Upgrade or via iTunes.

Cheers,

GB

It means what it says: in order to update the app you must update your iOS version.

Why would you not update?

It means you must update your iPhone to iOS 8 to install the Outlook app. That's apparenlty the minimum requirement.

Tap Settings > General > Software Update

An iPhone 4S can definitely be updated to iOS 8.

Disadvantage may be that the iPhone 4S is older hardware and may run slower on iOS 8.

Some apps you have currently installled may not be compatible with iOS 8.

Definitely backup your iPhone PRIOR to updating > How to back up your devices using iCloud or iTunes - Apple Support

I Always fail to download other apps