__Edit :__
I tried to do some math, and found that the x2 and x4 rule is based on the estimation that there is 50 cards left in the deck. In reality, there is less than that, 47 (45 in Omaha) on the turn and 46 (44 in Omaha) on the river. So it's a bit less accurate in Omaha than in HE, but it's still quite close.

You can calculate the error (to add to the result of the x2/x4 rule) with the following formula :

__On the river (x2 rule) :__
Error = x(50-a) / (50*a)

__On the turn (x4 rule ) :__
x(50a+25-25x-a²-a) / 25(a²+a)

Where x is the number of outs you have and a=44 in Omaha and 46 in HE. You can see that if you have more outs, the error will be bigger and if there is more cards on the deck (that's what "a" represent), the error will be smaller.

Was a bit lazy to write the details, but if you want them I'll make an effort

PS : For some reason I couldn't edit my original post, so I just made another, sorry for the double post.