Well, 7 outs on the flop, we have a 1-(40/47)*(39/46) chance of hitting, or a 301/1081 chance (27.8%, rule of 4 = 28%).

Thats like saying 301 cards help, and 780 dont, so 301:780, which reduces to 1: 2.59.

For the 4 out draw though, using the same method we get 1:5.07--> 1:5 (16.6:84.3, about 16%)
The 5 out 1:3.91 --> 1:4 (20:80, 20%)
The 6 out 1:3.14 -->1:3 (or 25:75, ie about 25%)

So the approximation is only really valid for the 7 outter...but that depends on how picky you want to be about half a digit