This was not offered as a stock option but for many decades Cyclo Benelux offered a conversion kit for SA hubs to allow 2 or 3 drive cogs to expand the range.

The SA hub being used would require that the longer axle was used to handle a wider block and spacers are required to give everything the right clearance and make sure the wheel is centered.

Am not sure if this bike is a 4 speed or just has a 4 speed shifter on a three speed hub and appears to be Schwinn like.

A modern variation on this is to use 2 3/32 dished cogs and a modern derailleur to provide a dual drive system and I have used this quite a number of times... it works really well when you convert a modern bike to three speed as the other drive parts are also 3/32.