Section 4. Of the land conveyed to the City of Philadelphia, a tract of not less than 75 acres nor more than 100 acres, the location of which shall be determined by the City of Philadelphia, shall be set aside for use as the site for a United States Veterans' Administration Hospital but if final commitments therefor are not made within three years from the effective date of this act, the land shall be used as other lands conveyed to the City of Philadelphia by this act.