My 3.75 yo daughter has a slight lisp which needs to be evaluated for tongue thrust. how difficult will the speech therapy be or will it fix itself?

A tongue : thrust is easily diagnosed & treated with an oral device by a dentist or orthodontist. If the lisp is frontal or interdental & she is assiduous about practicing what she learns in speech therapy, the combination would help correct or greatly reduce the lisp. A palatal or lateral lisp would be more labor-intensive to remediate, but can also improve with therapy.

Tongue tie: She may have tongue tie,the tip of tongue is held back due to a residual membrane under the tongue,if so ,this needs to be released,surgically ,have her see an ent first,then a speech therapist