Gluten forms as two proteins - glutenin and gliadin - link up with water and kneading. Full development takes roughly 10-15 minutes by hand or 5-7 by machine, so if you're testing at minute four, it's not failure, it's just early.
The test: pull off a small piece and stretch it slowly between your fingers, rotating as you go. At 100% development it stretches into a thin translucent membrane - you should see light through it - before it tears. Tears immediately while thick = keep kneading. Stretches thin and SEE-THROUGH first = done, even if it eventually tears at the edge.
That window matters because it proves the network is strong and even enough to trap gas. Tight tearing dough = gas escapes = dense loaf. Also, stretch SLOWLY. Half of all failed windowpanes are just impatient hands, not underdeveloped dough :)
