Get result for one column if data associated with that column is equal to two other columns

Question

I have data like this in Excel:

ColA         ColB        ColC           ColD
1B                         1B            E 
2B                         4B            C
3B                         5B            E
4B
5B

I would appreciate a formula to be applied in ColB for the following result:

ColA         ColB        ColC           ColD
1B            E            1B            E 
2B                         4B            C
3B                         5B            E
4B            C
5B            E

Answer 1

I think this may be much simpler than the possible duplicate suggested - ie just a very standard LOOKUP (which is duplicated elsewhere even MORE often!):

in B2 and copied down to suit:

=IFERROR(VLOOKUP(A2,C:D,2,0),"")

Answer 2

in cell B2, enter

=IFERROR(INDEX($D$2:$D$4,MATCH(A2,$C$2:$C$4,0)),"")

and paste down

Should do the trick!