I restored my iPhone 4s’ iOS8, forgetting to back it up on iTunes and was wondering if it’s possible to recover content such as messages, notes, apps, music etc? If it is, is there a free way of doing so or must I pay for it?
Agreed, most likely you won't be able to get the messages, files, notes, etc., back after upgrade/restore/downgrade, especially without having performed a backup prior. I've tried several tools to try to recover deleted files on android phones without any luck, but you may be lucky and recover at least some files with the Easeus tool mentioned above.
My general opinion on this is that you can only restore your data from either an iTunes or iCloud backup.
There are tools out there that (claim to) recover data directly from the iOS device e.g. EaseUS MobiSaver. I don't have any personal experience with those tools so this recommendation is just based on experience with other software from this company.