There is an issue with DVD Rot that is impacting numerous WB DVDs released from 2006-2008, and unfortunately, Lois and Clark is impacted. For L&C, the issue seems to be with Season 4, disc 4; my disc doesn't play.
This impacts a few hundred titles, more than just Lois and Clark, such as certain releases of Friends, West Wing, and Veronica Mars. It is limited to certain years and facilities when the DVDs were produced (there is a way to tell which facility your discs were produced). If you have the release from a different year or facility, it is not covered.
WB is supposedly giving replacements for impacted titles. Based on what I have seen, people are seeing mixed results with responses and replacements. You may need proof of purchase, like an image of the DVD box UPC (because who has 20 year old receipts). I haven't tried yet, so I cannot personally report.
Some good/bad news for L&C is that it has been released on Blu-ray, though unfortunately, it is missing some of the DVD features, the biggest being Dean's commentary on Season's Greedings.
Since the show is available on Blu-ray, they may be less likely to replace it, but you could try and get lucky, and there are many more titles impacted, so you may have other titles that have issues.
Here are some sites that explain the issue better than I could, including a google sheet of the known impacted titles.:
Video explaining the issue (it also has the link to the google sheet):
