SELECT *
    FROM mobilenum
    WHERE
        CAST(SUBSTRING(mobileno FROM -4) AS UNSIGNED) >= 1234
        OR
        CAST(SUBSTRING(mobileno FROM -4) AS UNSIGNED) < 1234
    ORDER BY ABS(search - CAST(SUBSTRING(mobileno FROM -4) AS UNSIGNED)

-- replace the 1234 with the last 4 digits of your search number, $search
-- ref: http://stackoverflow.com/questions/30932651