I have a standard NFC Ring and a Nexus 4 as well. The sweet spot on my Nexus 4 is pretty particular. I'm assuming you tested across the entire back side of the phone to see if there is an area with better NFC signal?

For my particular phone, the sweet spot happened to be (looking from the back) about an inch and a half from the bottom and an inch from the left side. There are other spots that will work, but that seems the best.