A circuit contains two light bulbs connected in parallel. What would happen to the brightness of each light bulb if two more light bulbs were added in parallel to the first ones?

The brightness of an incandescent bulb depends on the voltage applied to it. Here, we suspect the idea is that in a parallel circuit, all of the parallel elements have the same voltage applied.

If the bulbs are connected to a regulated voltage source, then the first two bulbs will remain as bright as they were, and the two added bulbs will go from being dark (when unconnected) to being the same brightness as the others (assuming all bulbs are the same).

If the bulbs are connected to a source whose voltage drops when more current is drawn, the bulbs will each assume a brightness consistent with the reduced voltage. (Usually, that means the ones that were on will get dimmer. The ones that were off (disconnected) will get brighter.)