For 4 tabs, a nested VLOOKUP would probably be ok.

You, can however use 1 VLOOKUP with an INDIRECT reference to the appropriate sheet.

eg.

=VLOOKUP(A1,INDIRECT("'"&B1&"'!A1:B100"),2,FALSE)

This looks in range A1:A100 in the sheet named in Cell B1 of the active sheet. It looks for the item in A1 of the active sheet in column A and returns the item in column B of the referenced sheetB