Quote from: Blizzard (Source)

Yes, the Season 3 honor items will still be available for purchase without a rating requirement after Season 4 starts.

There will be two sets of “non-set” items once Season 4 begins. The Season 4 honor items (boot/belt/bracer/etc) some of which will require a rating, and the Season 3 items (boot/belt/bracer/etc) which still won’t require any rating.

Just to sum up, when Season 4 begins:

Season 1 honor set: Removed

Season 2 arena set: Moved to honor, no rating requirement

Season 3 non-set honor items: Staying
Season 3 arena set: Arena rating and cost requirements lowered

Season 4 non-set honor items: Added with rating requirement
Season 4 arena set: Added to Arena with increased rating requirement