I personally don't like the mobilization limit idea. The US for example, was a draft-heavy military; its troops can be classified as more as militias than actual professional soldiers, and the National Guard was small. If the USA can be classified by Anti-War, or even perhaps Pacifist parties on the extreme end (The US placed a much heavier emphasis on 'soft' imperialism, or using SoIs than through war and territorial expansion - other than Mexico, anyway; bloody Manifest Destiny), it would by no means be able to bring up an army powerful enough for its size.
It would also mean that countries would maintain large standing armies for most of the period - while most armies were reserves, and were conscripted and mobilized in case of a conflict - a large standing army was costly. Four times would not be a multiplier large enough to represent these masses either.
And these are just army reserves, not even having touched conscription. Mobilizing should not be limited so much.
I believe a better idea would be to limit the total amount of troops mobilized per month, depending on party policy. A Pacifist country would be hesitant, and would not have had the systems or infrastructure prepared to mobilize large amount of troops at once, while Jingoistic countries would be very willing and ready to do so.
Another aspect might be the increase of WE due to mobilization - it would fit nicely with Jingoism's -50% war exhaustion boni as well, and I don't think pure economic malus is sufficient.
EDIT: If the mobilization per month idea goes through, it would also be nice if mobilization was a gradual process - there was one set large amount of masses you could mobilize, and you go through by them by percentage, and you could decide to stop mobilizing, or begin mobilizing more troops as the war continues - although demobilization would be only available at once, most likely at the end of the war.