I'm no expert but the just like avenseth12 said the problem could also be cpu/gpu related. We don't know what motherboard you have but most these days run on dual channel meaning it's preferred but not necessary that you use 2 identical sticks. Most are sold in units of 2 such as 2x 4gb or 2x 8gb . You could theoretically use 2x 2gb ones and one with 4gb ram but it's not a good solution. Better to buy 2x 4gb as ddr3 ram is pretty cheap these days.
Also, 4gb ram if that's what you have right now is too low for watch dogs. The minimum specs are 6 gb and recommended are 8.