Ok I've looked this up everywhere, can anyone confirm?
Does the DLC contained in the FNV: Ultimate Edition works on the xbox 360 Slim 4gb ? in the back of the box it says "hard drive required". However It should work on the 4gb flash because if you download the DLC thru xbox live, it gets stored there.

Your help is greatly appreciated.

I did some digging. Support forums for X-Box seem to confirm you can install all DLC content on the internal flash drive.. However.. I would strongly advice to get an hard drive. Among others so you can install the game it self.. saving on load times and giving you stability..

And let me stress (!!!) this is second hand info I got of two other forums.. so it is not 100% reliable!