Planetary scientists at the Southwest Research Institute have been running computer simulations of the early evolution of our solar system, and their models have suggested something kinda strange. When they ran simulations starting some four billion years ago with four giant planets like we have now (Jupiter, Saturn, Uranus, and Neptune), the resulting systems almost never ended up looking like our solar system does today. But when they stuck an extra giant planet in the mix, everything was ten times more likely to shake out like it was supposed to.
So it seems like five giant planets are pretty likely, at least based on these computer simulations. The fifth would have been an ice giant, probably similar in size and composition to Uranus and Neptune. But seeing as we've only currently got four gas giants left, what happened to the fifth? The simulations also suggest that at some point, the fifth giant planet would have had a not-so friendly encounter with Jupiter's gravity and gotten thrown out of the solar system completely. It's probably still out there somewhere, wandering through interstellar space, totally lost and alone. It's sad, yes, but it's something that astronomers are fairly certain happens all the time.