I've just noticed that as well... TEN MONTHS? That cannot possibly be right, given how promptly the other 2 seasons were released. By the time it is released, Season 4 will have pretty much finished airing. Hopefully we will see that date revised, because 10 months is ridiculous!
Because (in line with quite a few BBC series) they choose to release the DVDs just before the next season starts to air so the pre-air advertising for the new season also acts as advertising for the DVD release. This seems to be especially the case if that puts it in the 'last couple of months to Christmas' timeframe