It isn't going to match the iPhone in terms of speed.
As for is it better than the Nexus 4... You'll look around and you'll see that it is a pretty significant bump above the camera in the Nexus 4... detail, colors, etc... its a big improvement.
I'll say this... the Nexus 5 can take excellent pictures, but it can sometimes be temperamental.... particularly when it comes to focusing. Sometimes it would drill right in, nice and sharp and take a picture. Othertimes it would lollygag about, hunting for focus. It drove my wife nuts (we both had Nexus 5s) and it ultimately cause me to switch her over to a LG G3. She wanted a more 'point and shoot' expirience, so she didn't have a lot of patience during the times where it didn't exactly get its act together. I didn't have as many headaches because I knew its limits and was quick to tap my way to focusing correctly.
But.... it can, and will, take good pictures, better than the Nexus 4 most certainly.