Acircuit contains two lamps connected in series. what would happen to the brightness of each lamp if two more lamps were added?

In order to say definitely what would happen, we need to make sure that 
the light bulbs are the only items connected to the battery or power supply.
What I mean is:  You can't start out with, say, a string of Christmas tree 
lights, and at some point in the string, there's an extra little bulb in parallel 
with one of the lights in the string.  If that's the situation ... if there's anything else in the circuit besides the two light bulbs in parallel ... then we'd need 
more information in order to predict what happens when we make changes 
to that part of the circuit.

So the whole circuit consists of:  
Two light bulbs in parallel, connected to a battery.  That's all.

Now, we connect two MORE light bulbs in parallel with the first ones.
Now we have FOUR light bulbs, all in parallel, connected to the battery. 

-- The battery would see the total resistance of the circuit DECREASE.
(An electron that leaves one terminal of the battery now has 4 ways to 
reach the other side of the battery, instead of just two.  It's easier for 
electrons to get there, so more of them make the trip every second.) 

The light of the other 2 lamps will dim so power can go to the new 2 lamps