In functional changes there are two. Most noticeable is the enlarged magazine release that can be easily reversed for left handed shooters. To switch it only requires a small flat head screwdriver to manipulate the magazine release spring in and out of the necessary notch. Any Glock armorer should be able to do this in a minute or less. From a personal perspective, I really like this enlarged magazine release. It is much easier to manipulate in my opinion - especially with gloves on.
The second functional change was the replacement of the recoil spring assembly with a new dual recoil spring assembly. Both springs are captured so that there's little to no danger of the spring shooting across the room during disassembly (unless the plastic cap breaks off the front end). Having a dual recoil spring assembly does reduce felt recoil somewhat. You may not notice unless you take a Gen. 4 gun out with a Gen. 3 (or earlier version) gun and shoot them side by side using the same ammo.
What else is there to know? The Gen. 4 gun fits in every Glock holster I have. None of their changes affected external measurements except right where the backstrap is - and that has nothing to do with holster fit. Accuracy is as it has always been. From the 15 yard line, free hand, I was able to consecutively shoot 2" 5-shot groups. Many of the groups I fired were actually 1.5" or less. It was one of those times I wished I owned a Ransom Rest so I could machine fire it and measure the groups. I'm certain the gun is capable of greater accuracy than I am.
So, if you're a Glock afficianado, you'll be pleased with the changes Glock made with the Gen. 4 guns. If you're not an existing Glock fan, I'd recomend you try one out and see what you think. All jokes and odd nicknames for the weapons aside, they functional as they should. Load it, aim it, pull the trigger. It'll go bang every time unless you work hard to make it do otherwise.