I assume you know how to find the probability, p, of get a 6 at each roll.
If you do, you should know the probability, q, of not getting a 6 at each, roll, where q=1-p.
Once you know q, you can get to what you want by think this way.
To hit a 6 at least by 4 rolls means, you will not get it in the first 3 rolls, your probability of not getting a 6 in first three is .
Now, you should know the p.