why cant the gemorah be talking about a case of a lulav which already has all teh 4 nminim and then you add teh fifth so its not "gorua viomaid" why is it bdavka that you put another "min" together withn these 4 at teh same time?
Tosfos asks your question. He answers that "Mosif v'Gore'a" has to be *while* one is doing the Mitzvah. If he added the extra Min before he did the Mitzvah (even though the three were already together) it is considered "Garu'a v'Omed."
By Tefillin, writes Tosfos, it is possible to add the fifth Bayis when it is already on his head. By Lulav, on the other hand, once one has picked up the Lulav he has already been Yotzei the Mitzvah.