Gary, I'm in the exact same boat as you and have found it to be surprisingly easy in most regards. I switch the phones at least once a week and live in the SF Bay Area. There are only a couple issues that I have found thus far, texting being the biggest issue.
I did not change my data plan at all, I am still setup as an iPhone 5 5GB tethering plan.
I did setup two different APNs, the Phone one and the Cingular one (you should be able to search to find both but hit me up if you need the details)
I have a nano adapter and found that I need to roll up a small piece of tape to keep the nano sim in the adapter and easily put it into the Nexus 4. DO NOT PUT EMPTY ADAPTER IN THE 4!! I keep it in the small bag that the Nexus sim removal tool comes shipped with in my wallet when I use the 5.
Turn off iMessage before you remove sim from iPhone 5 or you will have messages going everywhere but your Nexus 4.
I turn both devices off during sim swap, I find it just picks up the network faster this way.
If you love using emoji symbols and sending pics while texting you can only use the Android Messaging app or the other messaging apps from the play store and the Cingular APN. I really don't know why yet, posed the question in the message boards with no replies yet.
Otherwise, enjoy! I found this to be the best way to enjoy both worlds, have newest hardware and least cost.
I am about 2 miles from Google HQ today (Mountain View, CA) and I ran speed tests on both APNs I use.
Name: ATT HSPA+ (this shouldn't matter)
APN: phone
MMSC: mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
APN type: default, supl, mms, hipri
Speeds: 108ms, 5.8mb, 1.1mb
Name: ATT
APN: wap.cingular
MMSC: http://mmsc.cingular.com/
MMS Proxy: wireless.cingular.com
APN Type: default, supl, mms
Speed: 140ms, 3.3mb, 1.1mb
I'm on WiFi a majority of my day so I tend to keep it on the wap.cingular APN because I like my emojis when texting, handcent functionality and ease of sending pics over text. When speed matters I switch back to the phone APN.
If you want to use any of the texting/messaging apps from the play store like Handcent or Go SMS and send emojis or pics then yes right now you have to use wap.cingular. If you like the stock android messaging app (I don't) you can use the phone APN.