To be fair , small plants do take forever to get going, they have tiny leaves 99 percent of light is being wasted at this point in that tent, think solar panels and surface area, we are solar light farmers with exponential growth as we develop more leaf area.
consider getting a tiny 2 foot x 1 foot reflective cloning tent with 40 -80 watts of fluro or , led bars. to grow the seedlings, mine grow 15cm high and root bound before they enter my main tent and are ready for 12/12 flower within a week. harvest ....and the new seedlings are strong and ready to go in to the 4x4... no downtime.
Ok what I would do, if you want fast growth you shouldn't over pot next time, by that i mean, start it in a small seedling cell 5 cm x 5cm and get it root bound first before up shifting to a slightly larger size pot, if a small plant is put into a too large pot relative to plant size (if it doesn't already have a strong root structure) it will stall and growth will be slow. number one,
second you blurple light is lacking seriously in light frequency and power , but as this is what you already have and if you dont want to or cant upgrade, i suggest adding a 4 x 150 watt white clf bulbs to the grow tent 1 hanging in each corner with the LED in the middle.
that might sound extreme to you or too costly but with out them in a 4 x 4 tent you wont be able to produce anything half decent im sorry with that 200 watt blurple Led light. ( it maybe cheaper to get a 600 watt HPS minimum ,ideally a hlg 550, even that does far better in a 3x3 than my 4x4)