I agree with Mr. Makkad's view here but would like to add that if the sketch, which i believe was the part of the sale deed as annexure, matches with the area that you are in possessoin of i.e. 500 sq. ft. than you can get a deed of rectification done. However, in the process as learned friends have suggested the cost of the stamp duty plays an additional role. There could be a situation wherein if you go explain the entire senario to your vendor and request him execute a deed of rectification, than he may ask for additional money since he himself is not the clear title holder of the property as he conveyed to you the property with the rights that he had been conveyed through his vendor. so it is purely a business call.
In any event, if the covenants in the sale deed says that you have been granted possession of 4200 sq ft, which is actually more than that, togetherwith all the rights to use the said property than as per my view you are holding this additional property subject to payment of additional duty for actual transfer of the said part of the property. However, you may appreciate that the possession of that part is only conveyed to you and not the title.
At this stage, you can also verify with the original approved building lay-out plans that what area has actualy been allotted and it will required to be ascertain that who was the actual title holder of that 800 sq. ft area and previous sale deed's will paly an important role to ascertain whether that property has been rightly conveyed with typographical error to you or not.
Hope this may help.