Would it slow down my iphone device if I update to ios 8.4.1?

Iphone 4s, ios 6 and 642mb available & 12.9gb used, how can I get more space? I have 4.4k photos. Any solution to update? I know your reaction is like 'wtf'. But please help me. I have no one to ask from and i don't know about these things. The other apps that I have are just the regular whatsapp, twitter, instagram, facebook, skype, kik, some games, etc. How can my phone be so full? Can I update, and how can I get more space to do it? I want to update, but should I?

also i don't know how iCloud works, it says total storage 5.0gb and available 4.7gb.

You've got a lot of big apps., and 4400 photos. That's why it's so full.

Upgrading to a new version of iOS will only make things worse in terms of storage. You simply need to get rid of some of the stuff on your phone. Use iTunes and offload some of those photos to your PC. Surely, you don't need 4400 photos on your phone.