You have to test individual elements in your string array.
Code:
IF tempStr(0) = "a" then
   IF tempStr(1) = "b" then
      IF tempStr(2) = "c" then
        IF tempStr(3) = "d" Then
          goto loop
        End If
      End If
   End IF
End If
Unfortunately, PBP will not test the whole string as a single argument.

You could also use the wait modifier to wait until ASCII characters abcd
arrive, then receive the remainder of your data packet in the array.

SerIn2 pinin,16572,200,start,[WAIT("abcd"), STR tempStr\8\"#"]