Season 4 in Overwatch will start 21-Feb-2016 its confirmed , each season last 2 and half months after that there is a mid season for 10 days or so.
The new season started today so will have more info in about 2 months about the big Changes and Rewards coming to Overwatch.
About the CP points for golden guns there should be not a big diference compared to season 3:
If you compete in each season you will earn a portrait, spray and cp points.
For a very high season start you can try our placement service for overwatch