If you want to try and get 5 years out of it, focus on 1.) what you need now to do what you want to do, as well as 2.) getting now the things you can't upgrade later which you may need to - which means RAM and processor. Based on what you want to do (photo editing), I'd go with the 8GB of RAM. I run Aperture on my 2008 iMac and 2012 MBA - you need at least 6GB of RAM to have a good user experience with Aperture. I would imagine that LR is similar. Otherwise, it bogs down when you are using brushes, working at 100%, cloning out stuff, etc. I upped the RAM in my iMac from 4GB to 6GB and saw major improvement in photo editing. So for me, 8GB is a must on a machine that you'll use for photo editing, unless you are only using iPhoto.
Next is the processor. The i7 probably isn't totally needed now, but 3-4 years from now it may be getting slow as image sizes will increase, apps will become more sophisticated, etc. The faster processor also speeds up importing/preview generation of photos, especially if you are shooting RAW. This is a good review for reference that compares the different processors as well as benchmarks against MBPs, older MBA's, etc.
http://www.anandtech.com/show/6063/macbook-air-13inch-mid-2012-review
Finally is the SSD. I have the 256GB SSD in mine, and it has about 160GB left. Granted, there's 40GB of photos in various users' iPhoto libraries on it, but this will be your biggest issue. If you want to keep your iTunes library on it, or your various photo libraries on it, you will start to eat up SSD space FAST. If you don't mind keeping an external drive handy, you can keep some of that off of your SSD, but I would have trouble with only having on 126GB of drive space as a photographer, even if you were judicious in your image storage. The good thing is that over time you can increase your SSD size as that IS a replaceable part, and at least 1 reseller (OWC) has them for the 2012 MBAs now.
What I have also done is to move my iTunes library to a drive attached to my Time Capsule. It's available wherever I am in the house, and the connection speed is fine even for streaming movies. I haven't tried moving an iPhoto library there yet, but I will in order to try it. I would think that for shooting in jpeg it would probably be OK, but it may bog down a little it with large RAW files over my wifi network (which is b/g/n).
You could definitely get by now with the 4GB/126GB model, and may never even need the i7. But if you DO want to keep it for a while (versus getting on the "buy cheap, resell soon before outgrowing, buy cheap again" carousel), AND ensure a good user experience with large files in LR, I'd seriously consider at least bumping up RAM now.
