Should i be concerned about a child who has suffered sexual abuse asking to use the restroom very frequently (4yrs old)?

Yes : The child could have a urinary tract infection or possibly a sexually transmitted disease. Please seek medical attention.

Possibly: Frequent urination may reflect infection, developing diabetes, anxiety and stress, fatigue (later in the day), overactive bladder. Pediatric evaluation is recommended to eliminate medical causes. A child who has suffered sexual abuse probably requires therapy to assist in coping, dealing with anxiety & stress. The child may be able to express reason and/or associated symptoms.

Yes: Any child with new onset voiding dysfunction or incontinence or any adult with lonstanding voiding dysfunction should be counseled about a history and nature of sexual abuse. Also, a physical exam is indicated. In children especially, problematic voiding habits can predispose to more serious sequelae as they grow into adults. Early diagnosis and treatment is crucial.

THERE'S MORE: It could be physical disease, but when your pediatrician has eliminated that, time for a child psychiatrist/psychologist to investigate.