Interesting question! I've never heard of a situation where only the perimeter (hairline) is super-curly with the rest of the hair being mid-3. I'm sure it's all around me but for some reason I overlooked it

Maybe one blending technique is to try the "tightly curly" method with your fringe: twisting those areas into coils with a finger while wet and with conditioner (and maybe even with gel or oil) in it, then letting those curls air dry like that so that when you put the rest of your hair in a bun/pony, you'd have some cute 4a/b coils framing your bun. ... Or something I've seen a few people with 4 hair do is use a quality butter or pomade to smooth and/or stretch those "edges" some.

For (especially) when you wear your hair down, you could try looking for styling products that tend to work well on curly hair in general. For example, many curlies (including me) swear by KCCC, which seems to play out especially well with a lot of curlies whose curls are in the 3 and 4 range. (Some, especially in the 4 range (but also some 3s), seem to rock Eco Styler, too). You might also consider searching youtube with related search terms to see if someone else has posted advice about this
3b/c?

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