The fourth-numbered member of the Phantom Troupe was Omokage prior to Hisoka joining. However, it is worth noting that there are some issues with this. For one, this happened in the Phantom Rouge movie that is not directly considered canon to the manga. As such, it is possible that you could believe that Omokage is not the actual member of the Troupe before Hisoka. That is completely valid but ultimately pointless at this time. It does not look like the manga has any intention of addressing this topic anytime soon so it is the best answer we have for now.