ABCD = 1000A+100B+10C+D
+ ABC = 100A+10B+C
-------------------------------------------------------------------
DCDC = 1000D+100C+10D+C
A=B=C=D=0
ABCD = 1000A+100B+10C+D
+ ABC = 100A+10B+C
-------------------------------------------------------------------
DCDC = 1000D+100C+10D+C
A=B=C=D=0