Help with Nexus 4

I recently got a used Nexus 4. Its rooted and runs 4.4 but when it boots Team Win Recovery appears and then it boots the 4.4 rom. Also, I noticed that I have only 6GB storage left on a 16GB model. With a file manager I found many files, probably trashes? But I don't know which of them it is safe to remove. As an iPhone user for years I don't know much about flashing roms etc on Android and I want to restore the device to be like when its bought by Google. I want to be able to get OTA updates, having a clean device etc.. maybe its the iOS syndrome? :p

Anyone knows a simple way to restore it using a Mac??