You'd have to look on the Addendum itself for the dates. If you have the otherwise-current API-650, you should be able to download the Addendum 4 for free (although it is in fact, an entirely new API-650- every page, not inserting pages). You'll have to contact API for this.
Implications- Wind design is changed, allows thinner shells but requires anchorage in more cases; seismic is all changed up, and requires seismic design in wider areas; some new material such as vacuum design added.