How to Add 4 digit numbers and display the sum in Assembly using one variable?

Question

I am using EMU8086. How do I add 4 digit numbers and display the sum in Assembly using one variable?

DATA SEGMENT
     MSG1 DB "ENTER NUMBER WITH FOUR DIGITS : $"
     MSG2 DB 10,13,"RESULT : $"
     D1 DB ? 
     D2 DB ?
     D3 DB ?
     D4 DB ?
     SUM DB ?
     RES  DB 10 DUP ('$')
DATA ENDS
CODE SEGMENT 
    ASSUME DS:DATA CS:CODE
START:
      MOV AX,DATA
      MOV DS,AX

      LEA DX,MSG1         
      MOV AH,9
      INT 21H     

      MOV AH,1
      INT 21H
      SUB AL,30H
      MOV AH,0
      MUL AL
      MOV D1,AL

      MOV AH,1
      INT 21H
      SUB AL,30H
      MOV AH,0
      MUL AL
      MOV D2,AL

      MOV AH,1
      INT 21H
      SUB AL,30H
      MOV AH,0
      MUL AL
      MOV D3,AL

      MOV AH,1
      INT 21H
      SUB AL,30H
      MOV AH,0
      MUL AL
      MOV D4,AL

      ADD AL,D3
      ADD AL,D2
      ADD AL,D1      
      MOV SUM,AL 

      LEA SI,RES
      CALL HEX2DEC

      LEA DX,MSG2
      MOV AH,9
      INT 21H

      LEA DX,RES
      MOV AH,9
      INT 21H 

      MOV AH,4CH
      INT 21H     
CODE ENDS
HEX2DEC PROC NEAR
    MOV CX,0
    MOV BX,10

LOOP1: MOV DX,0
       DIV BX
       ADD DL,30H
       PUSH DX
       INC CX
       CMP AX,9
       JG LOOP1

       ADD AL,30H
       MOV [SI],AL

LOOP2: POP AX
       INC SI
       MOV [SI],AL
       LOOP LOOP2
       RET
HEX2DEC ENDP
END START

Answer 1

The question you ask does not entirely correspond to the code that you've written. If you were to add a couple of 4-digit numbers then you would of course have to input more than one such number! See the EXTRA section.
Therefore I conclude that you maybe just ill-phrased the question and I'll comment on the code you've shown:

---

Your program adds the 4 digits of a single 4-digit number. This means that the maximum result can ever be 9+9+9+9 which gives 36. This fact is important for the rest of the program since it effects the choice of instructions.

When you actually input the individual digits you immediately destroy them by squaring them using the mul al instruction!

MOV AH,1
INT 21H
SUB AL,30H    ; From "0".."9" to 0..9
MOV AH,0      <-- Don't do this
MUL AL        <-- Don't do this
MOV D1,AL

There's no real point in moving the sum to the sum variable since it won't be used as such afterwards. But you do need to extend the AL register to the size of AX since the conversion routine expects AX as an input:

ADD AL,D3
ADD AL,D2
ADD AL,D1      
MOV SUM,AL    <-- Maybe not needed?
CBW           <-- This clears AH (the high byte of AX) because AL is a very small number (less than 128)

LEA SI,RES
CALL HEX2DEC

I see you've chosen to have the conversion routine HEX2DEC to always display 2 decimal digits. This is fine.

---

EXTRA

To set you on the way for adding a couple of 4-digit numbers, I'll add the code to retrieve the first of these numbers. Make sure to define word sized variables in the DATA segment.

Calculation is based on this formula: Number= D1*1000+D2*100+D3*10+D4

Summing both numbers and displaying them should be a breeze!

NUM1 DW ?
NUM2 DW ?
SUM  DW ?

...

MOV AH,1
INT 21H
SUB AL,30H
CBW
MOV DX,1000
MUL DX
MOV NUM1,AX

MOV AH,1
INT 21H
SUB AL,30H
CBW
MOV DX,100
MUL DX
ADD NUM1,AX

MOV AH,1
INT 21H
SUB AL,30H
MOV AH,10
MUL AH
ADD NUM1,AX

MOV AH,1
INT 21H
SUB AL,30H
CBW
ADD NUM1,AX