I asked the same questions a few weeks ago and I got a lot of responses saying I should go with 8, if no other reason than to future proof. However, I ended up going with just 4. I do pretty much the same things you do- office, excel, powerpoint, word, safari, itunes, spotify, twitter, facebook menutab pro, remote desktop to my work pc and other basic applications. I haven't come close to utilizing all 4 gb. Most of the time I am using 2-2.5. I use my computer mainly for fun and for online classes (i am working on my masters in a business related field). I did try to "challenge" my computer a bit and open everything I would possbily ever open all at once... and it still just barely made it to using 3 gb. Realisticly, I would never open all those things at once either.
So, I think 4gb is great. I had been using an iMac with 2gb and it worked pretty well, but I did get the beachball (similiar to the hourglass on a windows pc) sometimes. I have not gotten the beachball on my 4gb macbook air and I have not had any page outs. I am not worried about future proofing because I don't think my usage habits will change and even if the OS takes up more space in the future I still have plenty of room.
Hope this is helpful!