There's no Block-4 on the official record, not yet anyways. However, as
@arslank03 found, there are some reasons to believe that one could be in the pipeline -- e.g., Pakistan's in-house AESA radar, EW/ECM and AAM programs.
Personally, I think the PAF will continue ordering JF-17s through the long-term -- even in incremental batches -- to gradually phase out the F-7PGs and Mirage III/5s (including ROSE variants). I can also see the PAF look at replacing the older JF-17s (Block-1s and Block-2s) with newer JF-17s, especially as there are some clear 'breaks' between the 1/2 and B/3 (e.g., hybrid FBW vs. 3-axis FBW; AESA radar compatibility; etc).
The PAF has also invested in configuring the JF-17 for the strike role via IREK, AZB, Taimoor ALCM, etc. It's clear that the Thunder's poised to assume a much bigger responsibility in air-to-ground/surface missions. You'd want to carry forward that work into newer airframes with even better radars and electronics.
Finally, even if the KAAN hits a snag (e.g., delays in the TR Motor engine development), we'll still see lots of other subsystems materialize and mature in the meantime, like the Aselsan TULGAR HMD/S, new-gen EW/ECM, the IRST and EOTS, manned-unmanned teaming etc. The PAF might be interested in applying these to its fleet sooner than later, so they could look at applying them to the JF-17.
So, there are clear reasons for a Block-4 and even Block-5 to exist. I think we could see a Block-4 in 2027-2029.