The 4th generation was an improvement on earlier versions. The new pokemon they introduced were way better then the new 3rd generation pokemon. The graphics were way better then the 1st, 2nd and 3rd generation. The game got more challenging, compared with the 3rd generation, where you could just easily catch a level 70 Rayquaza and just sweep the Elite four (except for Drake and Wallace maybe), but in the 4th generation you could only get a level 47 Palkia/Dialga (I'm still wondering why such an odd number).
Though I have to agree with the people that say the 1st generation was the best, because the screen moved when your pokemon got hit, and took much longer to get thrue without running shoes and just a slow, crappy bicycle.
For the people that like the original and who would rather not play such modern game, of course they wouldn't the 4th generation. But if you do like modern technology and faster gameplay, then a lot of people would vote for the 4th generation, or even 5th generation.