I am going to assume that you this 4% is from the seller (seller's concession) and from what I am gathering, both parties (you as the buyer and the sellers) have agreed upon the 4%. Now you are thinking you would like to have them pay 6% instead? There are a couple of points here regarding this. 1. If both parties have agreed to the 4%, you may ask for additional closing costs to be paid, but the other party is more than likely under no obligation to agree (since there is the 4% that is already agreed upon.) 2. If you were to receive 6% instead of the 4%, then certainly the seller could offset his/her loss by increasing the price of the home. Sometimes that is what needs to happen in order to sell the home.
I would certainly consult with your agent to see if it is even possible to negotiate that increase. Once you are past a certain period within the contract, you are typically bound to what you have agreed to up until that point. There is never any hurt in asking, but you may not be able to even negotiate that at this point in your contract. Again, speak with your agent and see what your agent suggests.