Do you have pictures? Maybe you can flat twist the 4a sections of your hair, possibly put the flat twist in a bantu knot and take it out when it's dry. And you can style the 3b section as you would normally. Maybe it will blend that way?
I'm thinking 3b hair in a wash n go and 4a hair in a flat twist out/bantu knot out. Or you could just flat twist/bantu knot all of your hair.

I do all of my styling on wet hair, maybe this will help?