It is not at all clear what you are asking.

If the question is, "Select four items from thirty of which exactly twenty are defective. What is the probability that not all four are defective nor all four are non-defective"

The probabillity that all are defective is

.

The probabillity that all are non-defective is

.

Now note that those two are disjoint events.

The probabillity that all are defective

**OR** all are non-defective is

.

The probabillity that not all are defective and not all are non-defective is

.