Stand on your lease (which 9 times out of 8 will indicate that it is an approximation so that when its shown your paying for more square feet than you have, you can't demand a reduction and refund) and demand they show you within that instrument where what they discovered makes two red ants difference to the terms of the instrument. You need a good real estate or contracts attorney on this one. Sounds like it will be worth the investment (especially since there is probably also a paragraph setting forth the award of attorney's fees to a prevailing party on a contract dispute)
A proper response would require a thorough investigation into the history and background of this relationship. The information provided above is just that, information, to be used as you see fit.
I agree with the above, in some cases even an expired lease like yours can be admitted as evidence regarding terms and conditions. I would make sure that the new owner is apprised of your expiring lease, then you have a decision to make as to what you wish to do. If you sign the new lease you will be accepting the new terms of said lease, if you do not sign the new lease and pay the 4% increase based on your old square footage calculation the owner will have at least a reason to file an Unlawful Detainer, I do believe in a court of law it would come down to what is the actual square footage, before you decide your fate, hire someone independently to give you a true professional opinion as to the square footage. Good luck to you, but I believe your argument under these circumstances will come down to who's square footage calculation is correct. And I agree, if you decide to litigate this, my first call would be to an attorney.
If I understand correctly, your existing lease will end soon, and a new lease is being negotiated.
It is reasonable for you to ask for details about how the increase in rentable space was determined. And it is reasonable for you to request, as a continuing tenant, a concession to ease your increased financial burden.
However, based on the facts you have provided, it appears that any resolution of this matter will come from the business, rather than legal, perspective. Unless the new rentable space amount is clearly unsupported by the facts, I see no reason why the number in the expiring lease would be binding on the landlord in the new lease. The new landlord may simply say that the space was calculated incorrectly by the old landlord for the existing lease.
This information does not constitute legal advice and does not establish an attorney-client relationship.