How do you get the fourth badge in Pokemon Crystal?

You have to defeat the third Gym leader, then head to the lady who lives in a house at the top of goldenrod to the right. She will give you the squirt bottle. Once you have that head to route 36-37 and use the squirt bottle on the odd looking tree, once you have done that you are in Ecutreak city, were the fourth gym leader; morty is. He uses ghost type pokemon, so use ground and physic type pokemon. I used my gravlar and butterfree! Hope this helped