Is US4620479 patent number still valid or can this product be manufactured free on the open market?
If the patent has expired already, then, it has become a public domain and anyone can use it. But always be cautious in using the patent. Don't assume that when a patent has expired you can freely use it without liability. Always make sure that the owner of such patent did not file for any continuation, another similar patent or improvement and let the original patent expire. Always do in depth research on protected patents to avoid liability for infringement.
This patent was issued in 1986, so it probably expired in 2003. But no, in any event, this patent is no longer valid.
From what I can see, you should be able to do with it as you please.
As a general rule, no patent would last more than twenty years. The rules have changed a bit over the years (including since that patent was issued), and there can be some exceptions in events where patents were, for example, delayed in excess at the hand of the USPTO. But in general, something that's over twenty years is probably expired.