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.