I was searching on Walmart.com and saw they had a listing for the individual Season 3 set with a release date of 2/25/2014. I asked Time Life on their Facebook page if this was accurate and if Season 4 was going to be released at the same time (since Seasons 1 and 2 came out together) and they said the following:

There is no release date for Season 4. But, the release date for Season 3 is accurate. Based on the strong sales since Season 1 and 2 came out, there was a definite demand for Season 3

(Edited to reflect the real date, which was 2/25, not 2/24)

This is good news for those who didn't buy the complete series set and opted to buy the individual seasons. Hopefully seasons 4 through 6 will follow.