USAF hasn't received a non-VLO combat aircraft (ok, a non-VLO "fast jet") since 2001 when F-15E deliveries concluded. The only people still buying F-15s, F-16s, or F/A-18s are those who aren't cleared to receive F-22 or F-35, or those who need deliveries before those VLO aircraft are ready. Ok, and (maybe) Israel.
Most of China's current inventory is decades younger than USAF's non-VLO inventory. Even without J-11D (or J-10D, which would be a rather more sensible project), China is going to have >1000 non-VLO fast jets buzzing around for decades to come. There is no need, once equivalent next-generation platforms reach production status, to add even more legacy airframes to the inventory.
China faces only high-end threats, unlike USAF (which, again, has not received a non-VLO fast jet in fifteen years now) or European powers, China has no interest in bombing third-world nations. As such China has a greater imperative to transition to next-generation VLO platforms as rapidly and completely as possible than US or European nations do.
The US Navy is still buying F-18s, China continues to build the J-10, J-11??, J-15, J-16, and probably will build more J-11Ds, so yes high end 4th gen aircraft are still in hi demand. Of course there are many foreign customers as well.
Yes, the F-35 is also in extremely high demand, and like you I believe 5th gen is worth the price of admission, but that doesn't mean high end 4 Gens are ready for the dumpster.