10 days have passed since all of four plants sprou...

Easy fix. You need to buy a PH meter and figure out the PH of the tap water. I can guarantee your PH is way too high. In soil a PH of 6.3-6.5 will allow your plant to better absorb the nutrients she loves. So PH meter and most likely you'll need some PH down from your local hydro store or online. Keep your nutrients low in the first 2-3 weeks. 👌

Quite a big pot for seedlings, you are probably overwatering and the roots are drowning. Let the por dry out

That first little bit of growth is always the slowest I find till about the third node then it starts to pick up. I’ve never seen or herd of bio tabs before but personally I wouldn’t use them till they get a little bigger, says your using soil so there at lots of nutes in your soil at this point. Soil should be good for at least a month. I would also recommend picking up a PH pen This is very important for nutrient up take to high or to low and she’s just not gonna grab anything outa the soil. I like to run 6.5-6.7 don’t cheap out on a ph pen ya get what ya pay for! And make sure you CLEAN after each use with cleaning solution and calibrate often I calibrate every time I use it, it will extend the life hand fold. And if your on town water grab yourself an air pump and air stones and a 5 gal bucket and let this fun for 24 hrs it will get rid of the chlorine. Good luck!