What is the best place to train your Pokemon by the fourth gym?
For Pokemon Ruby/Sapphire/Emerald, the fourth gym is a fire gym. So, train a water type Pokemon by battling against the hikers at route 112.
For Diamond/Pearl, the fourth gym is a water gym, thus train a grass or electric type Pokemon on route 212.
For Platinium, the fourth gym is a fighting type gym, thus train a flying or psychic type at route 214
For the rest, i won't go on.