You need to add 3 gallons of 6% bleach to get up to your shock value of 20ppm FC. Run filter with pump on low 24/7, backwash as necessary (when pressure climbs 8psi or so over clean). Test as often as you can and add bleach as necessary to bring FC back to shock value. Brush the pool after adding bleach.

As you report 3.8 and 0.8 , I presume you're using the 25ml sample when testing chlorine. You're reagents will last much longer if you use the 10ml sample (remember to multiply by 0.5).

BBB for clearing a dirty pool:
Bleach - Bleach - Bleach and Brush - Brush - Brush