A seedling that young survives from the nutrients supplied in its shell , this is a known fact, ok ,
yours is 4 weeks old , it is not yet capable of letting you no it needs nitrogen,
you have fed it already,
the plant is turning light green telling you its ill,
i have told you already that the same symptoms can point to different issues,
i dont want thanks for the replies i have left you,
i just wish they were not ignored,
hope it works out.
Ps , you would not need a new pot or new coco , you can take out the plant , shake off the roots , check them , add your bat guano and some perlite to help aireate your coco, put the mix back into the pot and replant, no damage will be caused , lift the pot and remember the weight unwatered, water the plant , do not water again until plant is light to lift before you watered , This will work , over nuting will cause you more issues.