Hi Byron,
The yellow circle with the number in it is the Bridging Classification
Limit of the vehicle.
In other words, the number 4 indicates the gross loaded weight of the
vehicle. therefore, the vehicle could only cross a bridge that had been
rated to carry loads of 4 tons or heavier. The Australian Army also use the
same classification system.
Hope this helps.
Cheers
Rod Diery
Kulin Western Australia
Austin Champ owner and 'would be' restorer
ROD_DIERY@onaustralia.com.au