Have I fulfilled the driver's right after I subscribed to the bus for four days and did not pay him, then later subscribed with him for a month, and was absent for four days during which I called him so that he would not come, or must I pay the fare for the days I was absent or for a full month's subscription?

The four days you did not go with the driver in the second subscription do not compensate for the four days for which you did not pay the fare in the first subscription. This is because the driver is entitled to his fare if he provided the agreed-upon bus, and those days remain a debt upon you. What the driver is entitled to from your first subscription depends on the agreement between you. If the contract was for a month, then the full fare is binding upon you. If it was for each day individually, then only the fare for the four days is binding upon you. So, ijarah (definition) (leasing/hiring) is a binding contract, and the lessee owns the usufruct. If he voluntarily abstains from using the usufruct, the ijarah is not invalidated, and the rent remains binding upon him, just as if he bought something and took possession of it then left it.