I am just about to buy a 4 day rail pass for France. It says it is valid for 4 travel days out of that month. It is unclear as to wether it is valid for say, the month of October, or if its valid for 31 days from the date of validation. Out travel dates cross over between the end of October and the beginning of November so i want to make sure its going to be valid for use in two separate months!
To answer the question you asked, it's one month from the date that you activate the pass at a train station. And now for the question, you did not ask: it is often not a good idea to purchase a pass for train travel in France. See http://www.seat61.com/Railpass-and-Eurail-pass-guide.htm#Should you buy a railpass or pay-as-you-go.
Thanks for the info! I was planning on buying a 4 day rain pass for France, are you saying this often isn't the best option?
Yes, that is what I am saying. It depends on when you are traveling and if you plan to take long distance high speed trains. Take a look at the link that I sent in my first response. It's long so just scan down the page when it opens to the sections entitled "Should I buy a rail pass or point-to-point tickets? and "Rail Passes in France: a warning about quotas."
Really appreciate the head up. I just calculated the cost of our journey by just buying the tickets as we need them and we spend almost half as much! Good to know!