Slow growth during veg – 4 weeks old and still tiny

I see 3 potential issues:

1. The light — cheap blurple LEDs are inefficient. A proper full-spectrum LED like the Mars Hydro or Spider Farmer will make a HUGE difference
2. The soil — regular potting soil is not ideal. Get cannabis-specific soil like Fox Farm or build a super soil
3. Tap water — check your pH! Cannabis needs 6.0-6.8 in soil. If your tap water is outside that range, you're locking out nutrients

Fix the light first — that's probably your biggest bottleneck.

Seconding the light comment. Upgrading from blurple to a good LED was the single biggest improvement in my growing. Night and day difference.

Do Not use Any Soil from Home Depot. Its complete garbage, every brand for every type of plant. Its toxic and hot af in Nitrogen. Trust me when I say "I've tried them all" and they all suck.

Mother Earth Terracraft or Groundswell from Ace Hardware
California Light Works Solar Extreme Series
Camco RV/Marine Water Filter from WalMart $20.00 (attaches to garden hose)

Like LEDMaster said, that bluprle light is a main factor. Blurple light makes the roots stronger and deeper and will definitely stall growth. I only use Blurple to stall growth if nighttime temperatures are not around 60 degrees F and I can put the plants outside.