This is a space that would benefit from some ingenuity to make the best use of what you have. There are actually toilet/ sink combos that might work or I think this idea would be very nice and I don't think it would take up any more space than what you have .

Good suggestion. I'll research toilet/sink combos and see if they're viable.

Why would you place the sink on the 4' wall if the toilet seat--or a new, comfortably higher, likely larger, more efficient one--will still take up the same area where the old one still stands? I'm thinking the room is so small ones legs would be in the way of the fixture?