Generally speaking, modules created for Puppet 3 are likely to work in puppet 4 without changes. Of course, that's not to say that all will, you will have to test and make sure yourself since the module author has not.
However, if a module on the forge does not support puppet 3 by now, that may be a warning sign that development has stalled or the module has been migrated to another namespace. You should attempt to discover if either are the case, so you can plan accordingly - there's no guarantee a module designed for Puppet 3 that is incidentally compatible with Puppet 4 will also be compatible with future versions of puppet 5, and a lack of bug fixes may be problematic as well. Something to explore, anyway.
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Rob Nelson