Programs will generally only use Hyper Threading (HT) if they are designed to do so. If a program is designed to use 4 cores, but not HT, then it will only use the 2 two real cores in a Core i3 CPU. However, Windows 7 & 8 background processes will use HT.
Assuming the same clockspeed, each core in a 4th generation Haswell CPU is almost 40% more power than a core in the Core 2 Duo / Quad CPU. But that depends on the program; performance improvement can be less than that.