The ram usage of a 64 bit OS is only marginally higher and not really relevant. A 32 Bit os will only be able to use about 3 GB of your 4 because it is limited by its 4 GB memory space which is taken up by Vram and other devices on your motherboard though this can change depending on these two factors.
When it comes down to picking between 4 and 8 GB of ram it really depends on your uses and considering your cpu you probably wont be doing anything worth having 8 GB but it will still give some performance increase when doing lots of multi tasking. It might be useful to have in the future and 8 GB doesnt cost much more than 4 GB these days so if you can afford it go for 8 GB and 64 bit but if you dont want to 4 GB and 64 bit should be...