You have to approach the builder to effect name transfer on each individual's name with the competent authority. First and foremost thing, you should verify if he had obtained approval for the fourth floor from the authorities, if he has not obtained then it becomes an illegal construction and likely to be demolished anytime if the same is not regularised by authorised.
The builder is responsible for this. You have not stated whether the property bought by you has been registered on your name, i.e., whether the builder executed any sale deed in your favor and got it registered properly?
If yes, then you may apply for khatha directly too on the basis of sale deed. If not then you may issue a legal notice to the builder demanding from him to get the sale deed executed and registered in respect of the property purchased and effect the name transfer of khatha on its basis.
This legal notice can be followed by a complaint with the consumer forum seeking relief and compensation to the loss suffered.