change your code to
HERE: JNB RI,HERE
      MOV A,SBUF
      acall check
      SJMP HERE

check:
      cjne A,#'A',two
      cpl P1.0
      ret
two:
      cjne A,#'B',three
      cpl P1.1
      ret
three:
      cjne A,#'C',four
      cpl P1.2
      ret
four:
      cjne A,#'D',main
      cpl P1.3
main:
      ret