Leaves turning yellow from the bottom up

That looks like a calcium deficiency. I've had Cal and N deficiencies (among others) and the two are easy to tell apart: with an N deficiency, the leaves simply fade to yellow, from the bottom up, and as they do they'll begin to eventually fall off. But it doesn't produce the rust spots that you have, which a Cal deficiency does. Either can show up in week 4, though this is calcium. Symptoms of Cal deficiency (google for pics to see a pure calcium deficiency): 1. Lower leaves contort and curl. 2. Yellowish-brown spots develop on leaves and margins. 3. Flower development is slow 4. Root tips may die back (which will be invisible in soil) 5. Stunted plant and diminished harvest. The good news is week 4 is still pretty early so if you jump on it now, it may not have much impact on your final yield. Step 1 is to check and correct any ph problems. Though there are other ways to add calcium, every big brand has a calcium-magnesium supplement (Cal-Mag, Cali-Magic, etc) because hydroponic growers need them and soil growers not growing in organic super soils may as well. I needed it in my grow with Early Miss autoflower that I grew in Pro-mix premium potting soil. It really needed a full nutrient schedule which I didn't give it, and I think I managed a decent harvest despite the deficiencies but not as much as I'd have gotten if I had diagnosed and treated them before the plant was already ready to flush (its drying now).

Hi buddy ;-) Your plant is great 👍👌 That below leaves turn yellow and fall off is normal don't worry :-) I wouldn't change anything about your plant, it is really perfect 🌱😍👍