TS2345 Argument of type (prev string) => string | undefined is not assignable to parameter of type SetStateAction 어쩌구 타입스크립트 오류

setBoxInput((prev: string) => {
    if (prev !== '' ) {    //입력됐을때
        if (prev.length < 4) {
            console.log(prev.length);
            return prev + '' + n;
        }
    } else {    //맨 처음일떄
        return n.toString();
    }
});

setBoxInput((prev: string) => {
    if (prev !== '' ) {    //입력됐을때
        if (prev.length < 4) {
            console.log(prev.length);
            return prev + '' + n;
        } else {
            return '';
        }
    } else {    //맨 처음일떄
        return n.toString();
    }
});