Rainforce are indeed very good in the rain and give good wear. But, they are a "touring" tire and therefore quite stiff laterally, whereas the snows on the back are likely quite squidgy. This will make the handling of the car quite disturbing, as the back responds differently than the front to lateral inputs. Our 740 was really wierd with the touring/snow combination, especially on the highway.

I would go with four inexpensive snows, on scrapyard rims if neccessary. Having your snows on their own rims saves the mounting cost twice a year, and you swap the wheels when needed yourself instead of lining up at the tire store.

Or consider new all seasons all 'round and avoiding serious snow altogether. The problem here is you may get caught some day, and also next fall you will have four not-so-new all seasons and be back at square one.

Four snows on four rims is the best answer and it will pay off in very few years.

Good luck. Tough call, good tires are steep but important.