Number 3 and 4 label the terminals of the bulb.

Terminals are those part through which the electricity is passed to the filament of the bulb. They are made up of metal because their work is to conduct electricity to the filament. Terminals are like gates, if you want to go inside the room there is only one way that is passing through the gate. In case of bulb, if you want to enter the current into the filament you must pass it through the terminals. Without passing through terminals there is no other way.