G-2 and G-3 have been replaced by GF-2 and GF-3. There is no GF-4 in the new GF introduction page of Cybergraphics' web site. And I did not see GF-4 on B&H's shelf.
There are other options for getting another grade of contrast when you need it.
Selenium intensification of the negative can usually give you up to a one zone boost in contrast. Depending on film and fixer used, Kodak RST 1+3 for 5-10 minutes should do it.
Another option is to print dark and bleach the print (check Les McLean's book for more on this technique).