That's all it was - a concept.
Bel Geddes was an architect and industrial designer. He knew nothing of practical engineering of either the marine or aeronautical variety. He was a key figure in the aesthetic movement promoting streamlined designs and design motifs (e.g., Art Deco).
He allegedly consulted with German aeronautical engineer Otto Koller, but Koller never delivered any credible specifications for the Air Liner #4. Bel Geddes produced many concepts of structures that represented encapsulated towns or cities - including entire cities that somehow floated in the air. Air Liner #4 was a levitating city in airplane form.
The point of this concept was promotion of an aesthetic principle rather than proposal of an actual workable design for a craft. The same could be said of his concept for a streamlined "whale" shaped ocean liner - an Art Deco re-imagining of the notoriously unstable and unreliable whaleback ore carriers once used on the Great Lakes.
This review of
Horizons in an art magazine sums it up ...
A "Stylist's" Prospectus
Douglas Haskell
Creative Art: A Magazine of Fine and Applied Art, Volume XII, No. 3: February 1933
http://www.oldmagazinearticles.com/Industrial_Designer_Norman_Bel_Geddes-pdf