That is a funny but tricky question. I think it is just natural that you company will have to take care of their employees to make sure that they feel comfortable and come back again the next day. (haha) I bet it is just natural to give you a comfy chair, water, office supplies, some biscuits and water…etc.

The serious way is to discuss it with Human Resources. A chair will not cause a company to pack up and liquidate anytime soon, hopefully. The real question is, can you? No? But there’s a slightly fiendish way…

You said that there are only 4 of you in the office, then it will be much better to try to ask for your boss about it with pure subtlety. Try to touch him from their human side. Maybe suck up to him a bit. That is what I usually do. It may start with a joke or a pun but you can really build up on it to take it seriously. You also can test the waters how generous he is! If this is successful, you might even get more than what the boss might give. Just do not give your boss too many options though, some bosses suck at choosing and choosing on time.