I don't know if you have large temperature swings from day to night.
If you do, and if you have time to play around, you could shade those spots heavily with some kind of lean-to. Then the soil would get colder each night, but not much warmer each day. That might keep the soil temp under 40F long enough for maximum stratification.
But I think you already have the best plan for 4-year-old seeds: sow and hope.
