I'm still experimenting. I'm in zone 5, Latitude of almost 37, altitude of 7200 ft (I'm on a mountain side). The low latitude gives me more light than many in zone 5, but the high altitude makes for cool summer and fall temperatures even before the frost date is factored in. I have tried planting seeds by the first week in September. It's too late for me at that point, nothing gets very big at all. So I have moved back to early August, and that is much better. I think late July will work even better yet. Last summer I was overcommitted and never did get seeds planted, but for the first time I found fall vegetable transplants at my favorite greenhouse. So I put in kale, lettuce, and Swiss chard from transplants, and had a few spinach and carrots come up from self-seeding. That worked great! But we need a lot more kale (it was our first try at kale, and we are sold!).

This year I am trying some cabbage that is supposed to hold in the field well after frost (of course I will protect them with WOWs, a pile of pine straw, and my low tunnel), listing 100-120 days from transplant; and leeks, also listing 100-120 days from transplant. Both of those will go in during my May-June planting spree when I put out the tomatoes and seed the parsnips and main crop (long season) carrots. The red cabbage and kale will be started in mid-June and put out in mid-July, which is when I will seed the "early" carrots, lettuce, and spinach for the fall and winter garden.

I have a very different climate than you, but it is interesting to see what others are doing with the 4-season thing.

Catherine