( by Lewis Sellers )
You will need to install script VMerge from Script Gallery.
Go to menu "Tools" > "Script gallery" and then search for "vmerge" and then install it.
After you have installed the script, put the following formula in Cell A2:
=unique(query(vmerge('Sheet A'!A2:C;'Sheet B'!A2:C;'Sheet C'!A2:C;'Sheet D'!A2:C);"select * where Col2<>'' ";0))