Hi,
I have been having the same problem too.
No issues with any other connections at all, including 2 constant iphones and an ipad mini. Just the ipad4 drops with heavy internet connection. I have been getting this infrequently since Christmas, but it was more apparent when I was using a National Geographic app call fotopedia when running the slideshow which grabs the photos over the internet.
I have sent the ipad to Apple who checked it out and said there was no problem. I am in futher discussions with them and they deny this is a problem.
When it drops I go into settings and although it sees my router it doesn't connect until I click it. If I close wifi off and then re-open it it links automatically, so there is definitely an issue with the ipad4 or OS 6.1.3 or both on my system.
I am using BT Infinity 2 with 37mbs download and get 7mbs on wifi to the iphone and ipad (when connected!).
So you are not alone!
Sorry for being selfish, but I wish you were.