Dear Shom,
If the building has got approval for G+3 and you are already aware that the 4th floor is illegal why would you even think about it. You should always run away from the so called "deals". The government will be well within their rights to demolish the property. Whether they do that or not is a big chance that you will be taking with your life time earnings.
Hope you do not go ahead with this transaction.
Regards,
Kris