Just wondering when I should transplant from these into their final 15l (4 gallon ) fabric pots? They’re 21 days old
They look ready now.
They’re ready
they look great…time to transplant
Thanks for your reply! 
One more thing guys, my plants are currently asleep and lights are scheduled to come in @ 6pm, if I transplant them then will the light be too much stress for them or should I wait till before their dark period tomorrow?
Go ahead and Transplant with the lights on, they will be fine 
Just be gentle…roots are important.
Depending on how big you want your plants to get, 4 gallon pots are a bit on the small side. I use 5 gallon mostly and 7 gallon once in a while. 4 gal is fine, you might just see you are watering more often.
BTW rule of thumb is if leaves reach edge of pot, time to transplant. I use solo cups for seedlings and transplant when a couple of true leaves show up. They will seem to sit for a time in the larger pot until they send roots out then really take off.