Complex sentence: The DC-4's tricycle landing gear allowed its fuselage to be constant cross-section for most of its length, so it could be easily stretched into the later DC-6 and DC-7.
Split into simpler sentences: The DC-4 had a notable innovation in that its tricycle landing gear allowed it to incorporate a fuselage of constant cross-section for most of its length. This lent itself to easy stretching into the later DC-6 and DC-7.