One rear yard with a depth of not less than 20 feet shall be provided on any zoning lot except a corner lot. The provisions of Section 23-52 (Special Provisions for Shallow Interior Lots) shall be inapplicable. The provisions of Section 23-53 (Special Provisions for Through Lots) pertaining to R4 Districts shall apply except that the provisions in Section 23-532 (Required rear yard equivalents) shall be modified to require 40 feet instead of 60 feet in paragraph (a), or 20 feet instead of 30 feet in paragraphs (b) and (c).