That's the modems fault in the 4s. The 4s modem sucks. I get 9-11 Mbps now on the iPhone 5 with the 9615 modem
The 4s is good up to 14.4 so expect half at best in real world. The 5 is up to 41 so can be up to 20. AT&T uses HSPA+ 21 so 10-11 is best case real world