3.9 gb available, not enough space to dl?

I realize this is old so OP may have moved on or solved his problem but I just had the same problem last night when I purchased the game. For the benefit of anyone with the same problem, here goes:

I read in a forum somewhere that in iOS devices you actually need double to triple the free space required to install a game. I don't know how flash memory is written on the iPad or how it installs things but I deleted a couple more GBs and presto, the game started downloading.