If it's not your prime Photoshop device, the MBA is excellent for smaller changes if your with a client, having a portfolio demo on the go, and being able to just create on the fly.
The 4GB limit will be a restriction, but it depends on the size of the project and how many layers you actually work with, so the degree it restricts you will be variable.
Up until CS5 went 64bit, Photoshop could actually only access 4gb maximum memory anyway - so if your using an older machine with earlier version of photoshop you may not notice any difference anyway.
In which case it will be more than serviceable, heck I've been full-time graphic designer for 12 years. I remember when I started I could go and have a cup of tea and sandwich in the time it took to do some things in early Photoshop and that was on my brand new £5000 mammoth workstation with lots of expensive scsi hard drives. So things certainly have improved over the years
However if you are using the latest 5 & 5.5, and have access to pool of memory more then it may indeed be noticeable.
HOWEVER the biggest concern if the MBA is your prime Photoshop device would have to be color accuracy of that 13" panel, where even a slight tilt changes the color, making accurate color judgment impossible.
Try explaining to a client that was the orange they wanted but it just looked different on your screen... They won't be happy
