It is probably the combination of those :
2 in series, then two of those sets in parallel.
This is because Red LED has ~2V forward voltage, so 2 of them in series will have 4V
3 batteries are 4.5V.. just enough for two LED in series. I would recommend resistor in series of both parallel sets, to limit the current (when batteries are fresh)
For more details see here:
http://www.kpsec.freeuk.com/components/led.htm