I carry a Spyderco police most days. That being said, it has two drawbacks. The blade is slightly over 4 inches, if you get caught by a LEO you may have a problem. Second, the handle is stainless, so if your hands are wet, its a bit hard to get a grip on. That being said, its made with excellent VG-10 steel, and is very thin, making it carry very easy. There are much better tactical / everyday carry knives out there, but we all bond with a certain one.
I would suggest looking into the Cold Steel Recon (and AK-47) series, they are exactly 4 inches, have very good G10 handles that you can get a grip on in pretty much any situation. Best of all the lockup on those knives is incredible, its just like a fixed blade. The only thing I do not like about Cold Steel knives is the aggressive G10 handles that give you such a good grip also interfere with your pocket clip. Instead of smoothing out the area under the clip, Cold Steel leaves it textured, making it very difficult to get on and off your pocket. If you get any of their G10 knives, be prepared to sand down that area. Just make sure you wear a good quality respirator, as G10 is fiberglass.
As for serrated, it just depends. I have some tactical knives that are fully serrated just for the intimidation factor, and they will mess someone up if you have to (or yourself, if they get the knife from you). But for general EDC, serrations can be difficult to sharpen, and if you are needing to make a smooth straight cut in something, serrations wont do it.
I'd also suggest while in your knife searching quest, look for a knife with a full flat grind. Full flat cuts through pretty much anything with ease, and looks very good. If you have the option in the blade, I would go for VG-10 steel over AUS-8. VG-10 has very good edge retention and corrosion resistance, probably my favorite steel.
