How long can the ground be dry after spreading out new fescue seeds?(Will 4 days be too long after 16hrs rain)?

You are over-thinking the whole process. The soil is fine, plant now. Rake the sole a bit to rough up the surface, spread the seed, and it will grow just fine. You'll see the first grass in about 10 days, and it will thicken as it grows. If you notice bare spots, rough the area slightly, and reseed. Good Luck