@bindoy - depende yan sa design ng mobo. Some mobos have slot 4 free naman, it all depends on the IRQ allocation table included in the manual. Usually, Asus and MSI have these tables, pati ata Gigabyte has this. The more 'affordable' mobos do not have this data kaya you will have to go through the process manually, meaning, install the card and drivers, then check in Device Manager if it doesn't share an IRQ.
Do not rely on windows telling you that no conflicts are occuring since it will usually say that everything is ok even if the card is sharing. Some cards are more tolerant of sharing, but it's better to be safe than sorry in my book.