Sorry for the delay as I was travelling.
It appears that you have not purchased the land. The builder seems to have purchased agricultural land and he has signed an MoU with you wherein he has agreed to allot you a small part i.e. 4 Gunthas at some future time after he has converted the land in to NA.
Are you an agriculturist? If you are not an agriculturist then you cannot buy agricultural land in Maharashtra. You have not yet provided sufficient information for me to answer the query properly.
For example, it seems you have bought 4 mor gunthas from the same builder and that also is agricultural land. You are unlikely to have a registered sale deed and may have soem other document. Can you please provide a copy of the same ?
To make agricultral land in NA, there are certain conditions that have to be fulfilled. Also, the NA permission is given along with the land use i,e. whether it is made into NA for residential or some industry etc. The charges are not as high as you have been asked. The official charges are very low but soem money has to be paid to "consultants" to get it done and this fess depend on the value of the land but is completely negotiable. Also, the land size has to meet the minimum criteria which your 4 gunthas cannot meet so you cannot convert only your 4 gunthas into NA.