Divide it into group A(A1,A2,A3,A4),group B(B1,B2,B3,B4) and Group C(C1,C2,C3,C4)

IF (#1)A>B,
(#2)A1+B1+A3>A2+B2+COMMON BALL=>A1+A3>B2+COMMON BALL GOTO #3
(#2)A1+B1+A3=A2+B2+COMMON BALL=>A4+COMMON BALL>B3+B4 GOTO#3
(#2)A1+B1+A3<A2+B2+COMMON BALL=>B1<A2(WHICH IS EASY TO MEASURE #3)
(#1) A=B
(#2)C1+C2>C3+COMMON BALL GOTO#3
(#2) C1+C2<C3+COMMON BALL GOTO#3
(#2)C1+C2=C3+COMMON BALL=> C4 IS THE DIFFERENT BALL

(#3)FOR EXAMPLE: C1+C2>C3+COMMON BALL
AT THIS TIME, WE KNOW, IF A1OR A2 IS DIFFERENT, IT MUST BE HEAVIER THAN COMMON BALL, IF C3 IS DIFFERENT, IT MUST BE LIGHTER THAN COMMON.
THAN MEASURE C1 AND C2, IF C1>C2, C1 IS DIFFERENT, C1=C2, C3 IS DIFFERENT, IF C1<C2, C2 IS DIFFERENT.