About half way down this page...
http://www.adobe.com/devnet/flashplatform/whitepapers/roadmap.html
...under the heading 'Flash Player 11.8 and AIR 3.8' it says there is support for 4096 x 4096 textures and support for rectangular textures. Given that '2048' and 'square' were the restrictions for Stage3D textures I'm assuming this new support refers to Stage3D.
I have followed the instructions on this page...
Exporting flash cs6 to flash player 11.8
...(made a copy of the FlashPlayer11_4.xml file, altered it and resaved it as FlashPlayer11_8.xml, downloaded the 11.8 playerglobal.swc and switched to targeting 11.8) but when I try to include a 4096 x 2048 texture it still throws an error, saying the maximum size is 2048. Is there something I'm missing?
I'm using Flash CS6.