1. Ash, Pikachu, along with Brock, May, and Max, are in Hoenn, the land in the R/S games
2. Misty is not with the group, she had to go back to Cerulean to take care of the gym right before Hoenn started. She does come back later for a few episodes, though, won't go into details
3. Team Rocket is still there, but no Arbok or Weezing, along with Team Magma and Aqua, rival badguy teams from the R/S games
4. It depends on where you live if you want to see the new episodes, if you live in America the new ones come on the WB on Saturday mornings, usually at 9:00, but it is different in different places.
Ask away if you have more questions :)