Police Quest 2 is actually my favorite of the series. I think the 4th one is the weakest. Mainly because with the original three Police Quest game,s even though they did try their hardest to be like an actual cop simulator, were at their heart, still adventure games. They shared common things between the series (Soony Bonds, Jesse Bains, Lytton, etc...), and the character of Sonny Bonds was a very likeable character.
The 4th one, not being a Jim Walls game, lost what I call "The Sierra Feeling", which is that feeling you get when playing a classic Sierra game. I don't even think it took place in Lytton, did it? I haven't played it since it came out.