Software to manage packaging all files related to a product (for hand off to customer)
It sounds like you're more interested in the release packaging than PLM.  Just putting all the files together isn't a bad solution.  It works for a lot of projects!  With a few caveats and policies, it can be a fine solution.  

I'd recommend that it start with a version control system, not a plain folder on disk. It should also have a well-defined, well-planned structure.  This can be changed, but I'd recommend starting every project with the same basic structure.  For four people, doing what sounds like standard embedded system design, I don't think this will get out of hand.  

There are other things that you'll want to control besides the release, however, and a bug tracking/feature request tool will be required somewhere in the system.  There are plenty of those available. I've used Redmine.   

I work at a company with 5000 employees, and while we use the PLM/PDM bundled with various tools as much as we have to, our day-to-day development and product management is done through a combination of a few Subversion repositories and Redmine.