You will be fine with 4gb for what you describe. I had a 2011 i5/4gb/256gb 13" MBA and it was fine even with powerful software like Final Cut Pro and Logic Pro. Also remember that 4gb was the max you could get in 2011, they only made 8gb available in 2012.
Now I upgraded over the summer to a 2013 11" MBA i7/8gb/512gb because I had the opportunity to sell the old machine and wanted to get the top spec since I use pro software. But side by side, the 2011 and 2013 models really felt the same when doing things like mail, web, MS Office and Finder operations. Even Photoshop felt pretty much the same.
The main difference I see in the new machine is faster Final Cut Pro rendering and less latency in Logic Pro. So honestly, I don't think you will see any difference since I was comparing a 2011 i5/4gb MBA and the 2013 machines are already perhaps 30% faster.
Now 11" vs 13" is a totally personal thing that only you can decide. But after 2 years with the 13" model, I wanted the 11" and am now really happy with it.