Hi

Assume your data in A2:A14,

In B2 and copied down,

=IFERROR(INDEX(LEFT($A$2:$A$14,4)&RIGHT($A$2:$A$14 ,4),MATCH(0,COUNTIF(B$1:B1,LEFT($A$2:$A$14,4)&RIGH T($A$2:$A$14,4)),0)),"")

Array entered.