I had to work for this. Damned TV Guide killing TSOD destroyed a valuable treasure trove of news.
Seasons 1-4 (and 5-6, too, but those aren't part of the question) were originally released MOD.
Now, I can't say for certain there wasn't a pressed release of S1. It has always been cheaper than the rest, so if you say you bought a copy from Wal-Mart at one point, I'm not going to doubt you because I vaguely remember there being a pressed S1 at one point. But the rest were released MOD only, so what you have now likely aren't legit releases. And if this S1 is single layer, it's probably a bootleg, too.
The dvd-mod versions of seasons 1 to 4 are allegedly DVD9 two-layer discs.