ok well your half right, your didnt add any nutes or additives but FFOF and FFLW both contain nutes, its not slow release but rather organic forms so the micro organisms can break it down, i too use FFOF and a Happy frog instead of light warrior, i also use the GO line, i love the stuff but remember if it says 10ml per gal thats 10ml per gal dont try and sneek another few drops in there, especialy since you stated with a good soil mix. i wouldnt have fed untill the seed leaves were yellow and drooping which for my soil mix of 50/50 FFOF/FFHF is around 3-4 weeks depending on strain. also to note i do follow the the GO feeding chart to the T, EXCEPT for the start up. i start with 1/4 strength and work up to full strength depending on how the plants respond, usualy its 1/4 nute - water - 1/2 nute - water - full nute - water...etc. and from 1/4 to full, rotating nutes and water in a cycle, with my 2 day wet/dry cycle i can get to full strength without over feeding in just over a week, you gotta work them into the new nutes, cant just start full strength with such a small plant, speaking of small plants those last pics you shared are... well... quite small and dont look like they are 4 weeks old, they bearly have their 3bladed leaf set out yet, for one thing if they are 4 weeks old and still in those cups, transplant asap, i only keep mine in cups for about 10-12 days tops then into their veg pots, the small cups in my opinion dont have enough space for the roots to really develop properly to handle full strengh nutes especially starting with a nice hot soil mix. and another thing with bearly having their 3 bladed leaf set developed yet AND the seed leaves still a vibrant healthy green they havnt reached the point where they are big enough for anything but water, feeding nutes when the seed leaves are still healthy green is only asking for over nute issues. let the plant do its thing and it will tell you when its starting to get hungary, by yellowing the seed leaves, the food storage for the young plant while the plant and roots develop to the point they can handle feedings above and beyond the contents of the soil. dont feed before they need it, and again if those are 4 weeks old and still that small in those cups, transplant asap, at 4 weeks old no plant should be that small, there has to be a root binding issue there. autos or photo period, even northern lights the slowest vegging strain iv ever seen was about a foot tall and starting to bush out by 4 weeks old, and my autos currently growing were over a foot tall and had dime sized buds already at 4 weeks old.
sorry for the run on, hope it helps, major points, feed when needed NEVER before seed leaves yellow, starting with a good soil mix will prolong the time before feedings are needed, and in my opinion 2 weeks tops after seed breaks soil transplant out of cups and into vegging containers.