The-Numbers.com has now posted Blu-Ray sales charts für the last week of July. According to the chart the fourth season was able to sell 12,362 units for a total of $741,613 in it's first week. You can see it here: http://www.the-numbers.com/weekly-bluray-sales-chart
Correct me if I'm wrong but isn't that kind of bad considering what Season 1 sold in week 1 (also from the-numbers.com):
The best new release of the week was Star Trek: The Next Generation: Season One which sold 95,000 units and generated $5.73 million in revenue. This revenue was tops for the week. This is an excellent start for a TV on DVD release, especially one that's not a first-run release.
Seasons 2 and 3 didn't make it into the Top 10.