Regarding free SR servants

Unless you mean the welfare servants, there are sporadic "events" (usually X million downloads) where they give you a ticket which you can trade for any one of a select group of SR servants. This happens once annually I think. This year's has already passed, next years will happen in the latter half of 2019 and will have a better selection to choose from (all non-limited servants, including story-locked ones).

Here is a reference and list of available servants if you are interested:
https://fategrandorder.fandom.com/wiki/10M_Downloads_Campaign#SR%20Servant

To answer your 1st question, yes there was an event that gave out a ticket for a free 4 star and it has passed. And for the second question, yes there will be future tickets which you will be able to claim a free 4 star, with the download milestones set for NA. However just to set your expectations on what servant you may want, since NA clairvoyance has been nerfed some servants may not be included with those free 4 star events.