Electricity is like water, it will take the shortest path to ground. If in the same circuit you have bulbs with different wattage ratings it could add to the brightness. Have you ever noticed when a turn signal bulb opens (technical term for burns out or goes bad)? The signal will blink faster because the same voltage is being applied to less resistance. I would try putting the same bulls in all four sockets. If the problem persist check and if necessary clean all the connections. A little corrosion can cause havoc.
Good Luck