Most of the first-discovered extrasolar multi-planet systems were found to lie close to dynamically unstable configurations .
However a few observed multi-planet systems ( e.g . HD 74156 ) did not show this trait .
Those systems could share this property if they contain an additional planet in between those that are known .
Previous investigations identified the properties of hypothetical planets that would place these systems near instability .
The hypothetical planet in HD 74156 was expected to have a mass about equal to that of Saturn , a semi-major axis between 0.9 and 1.4 AU , and an eccentricity less than 0.2 .
HD 74156 d , a planet with a mass of 1.3 Saturn masses at 1.04 AU with an eccentricity of 0.25 , was recently announced .
We have reanalyzed all published data on this system in order to place tighter constraints on the properties of the new planet .
We find two possible orbits for this planet , one close to that already identified and another ( with a slightly better fit to the data ) at \sim 0.89 AU .
We also review the current status of other planet predictions , discuss the observed single planet systems , and suggest other systems which may contain planets in between those that are already known .
The confirmation of the existence of HD 74156 d suggests that planet formation is an efficient process , and planetary systems should typically contain many planets .