First you should fill the pot with dirt, not just half full. Fill it to about an inch from the top of the pot. Of course that means taking the plant out, adding more dirt, and basically repotting it back in. Don't just fill the pot up with dirt and cover half the plant.
Also if it is 4-5 weeks old it is time to start adding nutrients if you haven't already. Since you don't have a lot of dirt the plant probably used up all the nutes in that dirt pretty quickly. Also since you're adding more soil and if this is the first feeding, only give the nutes at 1/2 strength.
The plant looks pretty good otherwise, just get it repotted with more soil and feed if you need to.