At the start of week 4 sounds about right. But I like to judge by size, and also by when the last time I transplanted was. If I recently transplanted into fresh soil, then I would hold off on the nutes for another week or two.
By the way, what soil is that? It kinda looks like it has big chunks in it. I would try to get those out, they can cause the "bonzai" effect.
Good luck man happy growing!