You can have for of each different pokemon.
Darkrai G is different than Darkrai.
You could have up to 4 Darkrai G's in a deck, along with 4 Darkrai's.
Darkrai Lv.X is considered a Darkrai.
no, darkrai lv.x counts as his name being ddarkrai.
so you can use
4-darkrai ge/md(as long as they have the same name the set doesnt matter)
2-2 darkrai lv.x(stil adds up to 4 darkrai
however you can use 4 darkrai g and 4 other darkrai because darkrai g has a different name then just darkrai.
edit:double ninja'd
You can have a maximum of four regular Darkrai- non-SP. Meaning that you can have 4 Darkrais, 3 Darkrais and 1 Lv.X, 2 Dakrais and 2 Lv.Xs. And then you can also have up to four Darkrai G. This is because the G in it makes it a different Pokemon. Just by being from a different set doesn't allow you to have four of each in a deck.