We have no plans to ever use 4th generation sprites. Instead, we are currently in the process of re-doing every sprite, from scratch. Meaning we will be using 100% original sprites of the 493 Pokemon from generations 1 through 4. SEED, our graphics head has completed the front sets of the 493 Pokemon. Much more work still needs to be completed though, but they are beautiful.
As for the tiles, I'll say the same about the sprites. We are planning on having our own tilesets as well, so that the game is as unique as possible. The first couple of gyms are a little tough until your Pokemon can be leveled up faster. But it wouldn't be very fun if they were easy . They aren't impossible, but they are gym leaders after all.