The fourth era, lasting till the 1930s was the great steel heavyweight era. Steel cars of standard design and floor plan up to 85 feet in length were introduced in all the variations we have come to know.

Mechanically, the ventilation problem was finally resolved when air conditioning was introduced in the 30s and previous cars were retrofitted.

In terms of builders, the switch to steel caused a huge consolidation in the industry as small local builders had trouble competing. The leading builder of the day was Pullman but American Car & foundry had its supporters as did a few smaller builders. In the USA, most railroads were content to let Pullman supply the cars and operate sleeper and sometimes other first class services as well.  This drove standardization everywhere. Pullman was never a factor in Canada and the railroads bought (or often built) and operated their own cars. Here is the Pullman parlor car Lincoln Memorial built in 1930.