Can four tornadoes join together and make a big one?

Most likely not, as the chances of having two tornadoes close enough to merge is unlikely. However, some tornadoes, especially large, violent ones, somtimes have a series of smaller vortices inside the main vortex of the tornado. Under the right conditions these vortices can become visible, resulting in a tornado with multiple funnels. Tornadoes with four or more funnels have been observed. In some instances, these vortices will be visible for a time and then suddenly become hidden within the main funnel or debris cloud, making it look like they have merged.