2 cents, but deviate based on your more personal knowledge and details of what is going on and what was done previously -- raise light 4" and start fertilizing at 25-50% strength every other irrigation in soil. observe what happens.
i see it is an auto... could just be a small auto too. you got maybe 2-3 more weeks of vege growth coming... after that it all flower production. don't do anything too drastic. autos are easier in regard to sensitivity to light in dark cycle, but can be unforgiving bitches when it comes to maximizing growth with little experience.
Reasoning and more:
4 or 4.5 nodes counting the top?
you can fertilize before this point... if in soil it's more of a maybe depending on how hot the soil is...
it's difficult to know for certain if it is slow or just the natural vigor of that particular plant. genetic depression can cut vigorousness of growth significantly in just 1 generation... a few generations and it can really slow it down.
i don't think it's too little light based on internode length (distance between growth nodes is its own piece of anatomy). But if you want a litle more stretch, raise light 2-4"... early on you want to let the plant dictate intensity of light given... observe and react to internode length... ramp up to full power within 1-3 weeks or whatever the plant says... there will be variation, but similar each time.. take some notes about what you've done/did and review before next grow for any adjustments you'd like to try.
i don't think fertlization is a big factor to growth rate here, but could be wrong.. at the very least it looks healthy, which meansit's getting what it needs in some shape or form... doesn't mean you cannot give more, but should step up slowly and observe/react accordingly
irrigation == water until minimum runoff to ensure the entire volume got wet.. dry pockets are a potential problem of cuasing mineral buildup over time. wait fo top 1" to dry... learn weight of the pot as it's easier that sticking your finger into it and more accurate. Always water at similar weight and it will require a similar volume of water.. never start with "i want to give X liters pe day" that is the wrong way... you give what it needs and take note if you want to measure it out... some variance will occur, of course, unless you are weighing them on a scale, lol... don't need to do that.
if it's a water-only irrigation, you don't have to worry about runoff so much... any time you fertilize a little runoff is a good idea in soil.
i'd use close to 50/50 soil/perlite mix.. so if what you buy is a bit under that add some perlite, vermiculite or anything similar to improve drainage. this will increase irrigation rate, but that will likely have a positive result on yield anyway.. .can always use a larger pot if you want to slow it back down too. better root system = better growth. proper drainage, more easily infused wtih O2 from environment (naturally occuring with more gaps and such in substrate) will promote better and more root growth. (this is why clay balls are not a good choice for a substrate.. same reason you don't want big chinks of bark and other lumber in your soil.. impedes root growth.. yes they can grow around but it's some portion wasted energy and resources each time it does.)