As per your terms of contact you can resign. The same terms also state that you must pay the company the cost they have incurred in hiring you. They would adjust the same from your 4 days' salary. If that does not cover the total recovery due from you, then they may send you a letter asking you to pay the amount. If this fails then different companies have their own methods to recover including legal recourse.
You can wait for their letter and respond to it. Do you think you are justified to pay the amount they are asking you to pay? 2nd October 2016 From India, Pune