YANKEE TURNFLEX TURN SIGNAL PART NUMBER 730/6 4WIRES

A 4 wire switch cannot share bulbs with the brake lights, and needs separate turn signal housings installed at the rear. Maybe the front too if the parking lights cannot be easily converted to bright bulbs.

There is also no difference between front and rear on a 4 wire system. The four wires on a four wire switch typically are:

Power in (from flasher)

Right Bulbs (all)

Left Bulbs (all)

Turn Indicator (connects to 3rd pin of flasher).

Have you looked inside? Maybe some wires have been cut off? What is that knob under the control stalk? If that is for 4-way flashers, the switch is fairly new, probably 1960s or later. It kind of surprises me to see it as a 4 wire switch if that is the case. On the other hand, there were various conversion kits for older trucks that came with 4 turn signal housings to mount on the truck, and had no real need for a 7 wire switch. Maybe it's from one of those.

Her's a wiring diagram