Workspace with multiple projects or one project with multiple targets?

I have 4 iOS apps that are all similar. They're flash card apps, with each app having a different topic. They share a lot of classes and images. Right now I have all 4 of them as separate projects, but I'm looking to simplify maintenance and share resources. Would it be better to have all 4 projects in the same workspace, or have 4 targets in one project? What are some pros/cons of each method? They seem very similar.