Platinum is the best of the 4th generation of the Pokemon Games, but I do not think it was the true start of the 4th generation. Diamond and Pearl were full-fledged games, complete with a new region, new Pokemon, and were the first "true" Pokemon games for the NDS, true meaning not a spin-off, such as 'Pokemon Dash!'. Platinum was an upgrade of D/P, giving the fans more of what they want. Faster surfing, a battle frontier, and more plot-line.
And, as you all know, when a new generation of Pokemon games starts, it comes out with two choices between games. This is to spark trading between different versions and various other marketing reasons. D/P were the two choices of the 4th Generation. Platinum was the third installment, which, as said earlier, gives fans more of what they want, and introduces/fixes things. And with the release of the 3rd Generation, remakes of the first generation were made, FireRed (BPRE) and Leafgreen. In the 4th generation, to keep up with the pattern, HeartGold and SoulSilver are/will be released (depending on your local country at the time of this post).
Although the order is slightly off this time, the overall pattern is the same. Two Original Games, remakes (this was established in Gen. 3), and a third release. Platinum is the third release, in this case, making it not the official start to Gen. 4.
