Man, that is one ugly box-art. No wonder I never gave this a whirl in-era on the DS. Back in 2006 I found Pokémon Diamond to be an incredibly slow, molasses-like game, so for a long time it warded me off of the series. I didn't get back into the series until a decade later with Gen 7.
Similarly, for these posts there was a ten year break between the first three generations and this one. In the interim I did the two Legends games and the Gen 1 remakes, but that's about it. Picking up with 2008 here.
The good news is that Gen 4's deluxe/upgrade version Pokémon Platinum fixes a lot of the issues with Gen 4 and speeds things up quite a bit. This is EASILY the definitive version of Gen 4, and one of the best Pokémon games period. It started a bit of a renaissance for the series, being followed by Gen 5 (debatably the best one) and super-solid remakes of Gens 2 and 3. Platinum heralds a pretty huge turnaround considering how bad Gen 4 started.
Historically, Pokémon posts were always R-Rated and weird as hell. Read at your own risk.