Mine are a little bigger at 4 1/2 weeks under LED. CFL doesnt have as much power so yes, yours is a bit small for its age, but nothing out of the ordinary.
As for nutes, if your on a small budget, try out Dyna Grow. They have 2 ferts, a grow and a bloom. The grow is what im using right now at 7/9/5 and the bloom is 3/12/6. Each are 1 pint, or 473 ML for $12 each. Use 1.5 to 4 ML per gallon per watering. Thats 1/4 to 2/3 strength. This little bottle i can feed 5 plants through 8 weeks of veg and still have about half a bottle left when i flip to 12/12 and switch to bloom. I assume ill use more bloom, maybe half to 3/4ths the bottle.
I also got a bottle of 8/0/0 nitrogen boost for $18. This bottle is 3 times the size and uses the same amount, so i should see 3 or 4 grows out of it.
As for when to use nutes, well, your ready. Usually u can start nutes when the 3rd nodes start.