There are a few options
Excel 2003/2007 either
[F1] = IF(ISERROR(VLOOKUP(E1,$A$1:$B$4,2,FALSE)),"",VLOOKUP(E1,$A$1:$B$4,2,FALSE))
or
[F1] = IF(ISERROR(MATCH(E1,$A$1:$A$4,0)),"",INDEX($B$1:$B$4,MATCH(E1,$A$1:$A$4,0),1))
Excel 2010 either
[F1] =IFERROR(VLOOKUP(E1,$A$1:$B$4,2,FALSE),"")
or...