With 4 modules you may have to increase your memory voltage to compensate for the additional load. Try with 1.35V and then test with your 4 modules by gradually going up to 2400-2666-2800 and so on and see how far it allows you before becoming unstable again. If you are able to run them with the higher voltage at a ceratin frequency, then start bringing the voltage down again until it becomes unstable again. That should give you and indication what voltage will be needed at that frequency