Is the debtor sinful if he does not pay his debt, which its owner stipulated be paid within a month, even though the debtor will not have the money until after four months?

As long as you are in hardship and unable to repay the debt, the creditor is obligated to grant you a delay until a time of ease, for Allah Almighty says: "And if there is one in adversity, then [let there be] a postponement until a time of ease." (Qur'an 2:280). If you become solvent and able to repay, you must immediately repay your friend's debt unless he agrees to a delay, otherwise you would be unjust, for the Prophet, peace and blessings be upon him, said: "Procrastination [in paying a debt] by a wealthy person is injustice."