They may have been hedging their bets. Yes ST:SNW got it's S4 renewal order during S3 production; but at the time the writing room did the season 3 overview/outline of what stories/where, they didn't know they were greenlit for S4.
Which sounds to me like S4 will be the last, if they were worried that the show would be cancelled at the end of S3. How much production has actually started on S4?
There's also the possibility that at any time, Paramount could simply have them cease production. And if post-production isn't finished for SFA by that time, that's another problem. (Although I'm fairly certain it will be fine, but anything past the first season is now in doubt.)