What to do after you beat the fourth gym leader in pearl?

Answer 1

After beating the Fourth Gym Leader, you have to talk to the Team Galactic Grunt who is standing by the entrance to the Great Marsh. He'll run off so you have to follow him and keep on following him until he battles you. After beating him he'll run off and Cynthia will come by to give you a SecretPotion. This SecretPotion will help you to cure the Psyduck that are blocking a route and then you just take the route that the Psyduck were blocking and follow the route until you get to Celestic Town which is where you'll go battle and beat a Team Galactic Grunt that is there and then you can go into the cave that the Grunt was blocking and you can examine the painting at the back of the cave and after examining the painting, Cynthia's Grandmother will come in to give you the Surf HM.