Why not check the impact factor (or similar metrics) at a well-known, reputable source, and then compare it to the rest of the field? I've never heard about Scientific Journal Impact Factor (this one: http://www.sjifactor.inno-space.net/?), but it sounds suspicious and it doesn't list several well-known journals (at least in biology). The "original" impact factor comes from Thomson-Reuters, but in case you do not have access to it (you library should have it though), there are also free reputable services that offer similar metrics. SCImago journal rank or Journal metrics from Scopus comes to mind. Compare the journal you are thinking about with others in the field using one of these services. If the journal you are considering cannot be found there, I would consider this a red flag.
A journal ranking list in philosophy from SCimago can be found here. If you order that list by "Cites per document (2 years)" you will have a very close approximation to the Thomson-Reuters impact factor. From what I can see, an impact factor of >4 would be off the charts for a philosophy journal, which makes me even more sceptical about the metric that you have been looking at.
What's the name of the journal you are considering?