Can't update my iPhone 4 to ios 8 - Itunes can't find my update?

When I go onto itunes and click "check for update" it says I'm up to date and shows the version (7.1.2) so I basically can't update. I need ios 8 even if it's faulty to download certain apps.

I'm almost sure you can't download it as the iphone 4 is a old piece of hardware. Devices eventually can't download updates as they become obsolete.