After the signing of the treaty, there was no more war. (A specific war, a state of war was going on, and now it is over.)
After World War II, there were no more wars in Europe. (Speaking of unspecified wars in general.)
4) doesn't work without the plural for me- the plural suggests that there should be no more nuclear bombings like the one that befell Hiroshima. Without -s, it suggests to me that the city should cease to exist.