That one tire dealer may not realize that this is a full-time AWD vehicle. His advice would be ok for a part-time 4x4 vehicle but not your subaru. Many people fail to make this crucial distinction.
I would replace all 4 at the same time. Then keep up with regular rotations. Keep the 2 tires that still have life to them & either sell them on craigslist, or keep them for another vehicle - one with 2WD.