At three to four weeks from seed you should start seeing the bottom leaves turning yellow. This means the plant is using up it's available nitrogen and to a tiny extent, potassium and phosphorus. Nitrogen is always the first to be used up and it shows as an even yellowing of the leaves. Sometimes starting at the very tips, but quickly it becomes the entire leaf.
I've always used this sign as the plant telling me, okay go ahead and start feeding me now (or transplant into a larger container).