If i upgrade my iPhone 4S to iOS 9, will i loss my lose any of my data like photos, contacts or etc ? Even if the phone has not been backed up for the last few weeks ?

No you shouldn't loose any data upgrading, however i would always recommend making a full back up before upgrading just incase something does go wrong.

You should not lose any data but as you know, with any device made by man, glitches can rarely occur. I strongly recommend that you import the pictures as per below. All else can be restored in the rare chance that something goes wrong. I would also recommend that your do the update through iTunes and not Over the Air, you will get a whole new file set instead of just the changes and it is more reliable, less prone to issues.

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

Import photos and videos from your iPhone, iPad, or iPod touch to your Mac or Windows PC - Apple Support

Hi

Go to settings - iCloud - backup - backup now

Then update software to ios 9.1

Cheers

Brian (uk)