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:

Division      CP
Bronze       100
Silver         200
Gold          400
Platinum    800
Diamond    1200
Master       2000
GrandMaster    3000

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