What is the difference between 3rd Generation & 4th Generation Intel i5 processors?
There's a surprisingly large number of differences between Ivy Bridge and Haswell. In particular, the chipsets associated with Haswell (Z87 and other 87 chipsets, such as H87) have a number of advantages over the older Z77 chipset. This is the best comparison that I could find:
Essentially, you get a lot of features in Z87 that aren't available in Z77. I like the ability to quickly sync video between peripheral devices and the Z87 chipset. It's probably the most salient feature.
I'd like to add that Ivy Bridge was the first series of Intel CPU to use the so-called 3D (or Tri-Gate) Transistor technology, which was rolled into Haswell. There's big difference between 3rd and 4th generation Intel CPUs. The biggest is probably the Z87 chipset, which is probably the biggest upgrade in the features offered by chipsets in a decade.
Unfortunately, Intel chose to use thermal paste between the die and the integrated heat spreader, so Haswell and Ivy Bridge (not Sandy Bridge) CPUs will slowly experience temperature increases over their life span, shortening their operational life.
Have a look at the below article to know detailed difference between the two -
The Haswell paradox: The best CPU in the world… unless you’re a PC enthusiast
The 3rd Generation was primarily a die shrink therefor the processor remained mostly the same as 2nd generation only in a smaller package, slightly lower power consumption and better performance plus an updated graphics solution.
The 4th Generation is based on a new micro-architecture and comes with new features e.g. Intel MWT, improved multi-threading technology, boost in performance compared to same speed previous generation, lower power consumption reported as up to 50% increased battery life (tests show an average of plus 1.5hours) and as before a new graphics solution generation.
I think the deference between the 3rd gen and 4th generation processors is the power consumption, the 4th generation CPU's are better for laptops because they will extend your laptop battery life,
3rd and 4th Generation Intel® Core™ Processors for Business
It depends on if you are talking about the desktop processors or the mobile processor, but other than the different socket type, the difference is a slightly better speed, better power efficiency, and modest improvement in integrated GPU power. Haswell has been known to produce higher heat, though. Unless you are upgrading from an Ivy Bridge (3rd generation) to a Haswell (4th generation) laptop or other mobile device where battery life would be better, I would hold out on getting a newer Haswell system.
If you were looking to upgrade from Ivy Bridge to Haswell chips, you would need to purchase a newer compatible motherboard as well since the socket types are different. Even though the newer CPUs have a better integrated GPU, it still isn't worth the extra cost as it wouldn't offer that much better graphics capabilities. Native support for 1600MHz memory is available with Haswell, which is a good boost, but not enough for an upgrade from Ivy Bridge. If the price difference is small, go for Haswell, unless you plan to overclock the processor. Otherwise, save the money for a better graphics card if you are wanting one. For more information to compare 3rd generation to 4th generation (although the comparison is between different i7s instead of i5s), you can take a look here: http://www.tested.com/tech/pcs/455882-tested-intels-haswell-cpu-desktop/