I've owned the 4120B as well as the 4160B and 4163B, and I would concur that they are not the greatest for CD burning. Currently my 4163B burns with C2 errors on quality media like Fuji (Taiwan mfg) Prodisc and Sony CD-R media. This has been very disappointing, as I have used LG CD burners for years with very good luck.
I actually switch to Liteon for a while once I started using DVD's, because I had audio burning problems with my Pioneer DVD burners on CMC media. I began using Kprobe quite extensively and found it useful, but eventually started to find that bad kprobe results were Liteon firmware problems and not the actual burns. I was also tired of weird Kprobe drive lockups, and Liteon claiming my drive was defective and needing to be exchanged every time I pinned down a firmware bug for them to fix. I also found the Liteon Kprobe readings more "optimistic" about the actual levels of errors found, and their severe problems evaluating certain media (mainly low quality CD-RW media and low grade DVD media) finally made me stop using Kprobe for the most part. I picked up a modern generation DVD burner of their recently to check on their progress (1633S) but still found them to be lackluster drives despite their frequent firmware updates.
I did find LG more responsive in fixing bugs which I found and identified to them, but their media support is not as wide as the Liteons for bad media. I have not yet emailed them about the consistent C2 errors on the media identified above.
At that point I picked up a BenQ and the results are much more reliable, and I really find the jitter measurements valueable. I have found cases where two discs with similar C1/C2 readings fail to play properly, and the one which does play properly is the one with the lower jitter reading. I'm not sure whether you use your DW1620 for this, but definitely convert the drive to the B7xx firmware as the G7xx firmwares do not support error scanning as well. They are restricted in speed, and some firmware versions will not even report certain errors types. I have now purchased a 2nd DW1620 and it has showed the same characteristics. Another quirky feature of the DW1620 (B7V9 f/w) is that they only burn Verbatim 10/12x CD-RW media at 4x, but they burn 32x CD-RW media (which they don't really even need to support based on their specs) at 16x albeit with a terrible C1 index > 250 and jitter > 20%.