3. Discover that the same drug works for a different disease and patent the new application of the old drug.
4. Since the drug is still patented even after the original patent has expired, anyone making the drug still has to pay up even if they are selling it for the original, out of patent, use.
If you patent for disease A, then the patent expires, even if you patent for disease B, it doesn't stop anyone from making a generic version of your drug to treat disease A. Doctors will also prescribe that generic for disease B (there is nothing to stop them).
3. Discover that the same drug works for a different disease and patent the new application of the old drug.
4. Since the drug is still patented even after the original patent has expired, anyone making the drug still has to pay up even if they are selling it for the original, out of patent, use.