hi,
I mean not securuty but just an idea. Original poster has neither indicated chip he used, nor conditions of security.
So I suggested something like:

L_PERMISSION:
	CALL	SUB_GET_DIGIT
L_PERMISSION_0:
	CJNE	A,#PASS_1,L_PERMISSION
	CALL	SUB_GET_DIGIT
	CJNE	A,#PASS_2,L_PERMISSION_0
	CALL	SUB_GET_DIGIT
	CJNE	A,#PASS_3,L_PERMISSION_0
	CALL	SUB_GET_DIGIT
	CJNE	A,#PASS_4,L_PERMISSION_0
; autorization passed

Regards,
Oleg