If they support the iPod Touch 5 there is no reason not to support the 4S as they are the same internally in terms of the processor, ram and graphics. I assume since the iPod Touch 5 is for sale, that it will be supported, and thus the iPhone 4S will also be supported. Also the iPad Mini 1 is still for sale which is also A5. In fact between the iPad 3, iPad 2 and iPad Mini 1, around 60 percent of iPad users use a A5 iPad.
There is no real 'pattern' to iPhones receiving support. The 2G and 3G received 2 updates and the 3GS and 4 received 3. No reason that Apple can't give the 4S 4 updates, especially if iOS 9 is focusing on bug fixes and performance improvements (if the rumours are correct).
There are also far more iPhone 4s's in use than previous iPhone models in their 3rd and 4th years, as Apple sold more 4S's than any model before it.
Lets also remember that the iPhone 4S has done better than the iPhone 4, in terms of running its 3rd update far better.
https://www.youtube.com/watch?v=iU06MwGikwA&spfreload=10
Thats iOS 5 vs iOS 8, and tbh there isn't a great deal of difference.