Contract of Employment Terms
This Contract of Employment is terminable without reasons by either party giving 2 months of written notice or payment in lieu of notice. You shall serve a period of 6 months on probation.
If you have a term similar to this in your offer letter or employment agreement, then I don't think you need to pay even a single penny for his 4-day service. If the employee really has a serious concern at his workplace, he should discuss it professionally with HR or his head. On humanitarian grounds, if it's a genuine case, then he can be relieved with immediate effect, and even then there is no guarantee that 4 days' salary needs to be paid unless the deciding authority authorizes it.
Professional Workplace Expectations
Work is a professional place. After all the efforts of recruiters, interviewers, HR, and documentation for so many days, he or she can't just come in passing time for a week and leave without a reason.
But eventually, you would have a contract that would have a clause related to resignation or termination which decides if the payment needs to be done or not.