Hello,

The photo is of a Pennsylvania safety inspection sticker. Back then, vehicles were required to be inspected twice per year, The state used four cycles of inspections based on the last digit of the vehicle VIN number. The number four on the sticker in your photo is the inspection cycle for that vehicle.

See the attached for more details:

http://www.bufs-plates.com/windshield/pennsylvania/pa-ws-1969-1983.html