I started with Red, so quite a few years before gen 4, but I've always felt like gen 4 was under appreciated. While it's not my favorite gen, it's definitely up there, and my Diamond had one of my highest hour counts (I think somewhere in the 450-500 hour range; I've never maxed out the time).
I first got into breeding a little bit in gen II, and I learned about EV training in gen III, but I became a much heavier breeder in gen IV. All those hours biking up and down, watching the step counter (a feature I really miss)...I also got pretty into RNG breeding for shinies (though I never was able to figure out the timing for manipulating IVs via RNG), and I bred my first shiny, a Squirtle with a terrible nature, just over 10 years ago (I think it was 10 years ago yesterday).
I realized from a survey I filled out the other day that the plot and locations didn't stick with me as well in gen IV as in the earlier generations (some of the city names are a struggle), and I think I owe it another play through sometime soon. Still, the generation was memorable for me for the revolutionary changes it introduced, namely online trading and the physical/special split. I recall the latter being pretty controversial at the time, but in the end, I guess most people ended up being happy with it, and for most Pokemon, it broadened their movesets (except for everyone's ruined, now-physical Sceptiles). The former changed Pokemon from an increasingly isolated pastime for me (I didn't have many friends who still played) to something social, and it made for an interesting summer for me shortly after it came out (summer of 2007). Although I was away for some of the summer, I recall spending the time I was back trying to grab shinies in GTS giveaways on the NSider forums (no friend codes technically allowed, though of course there were ways around that), and I remember setting up a breeding shop and arranging deals for shinies and events.
I'm curious, for those of you who started with gen IV, if you've gone back and played any of gen I-III, what do they seem like to you? Do they seem clunky? Isolated without the online elements (even though online has been shut down for a while now for gens IV and V)? Having only watched the series evolve over time, I'm curious as to what previous entries seem like to someone who started partway along.