Fourth lake in Sinnoh

[Bobby2]

I'm unable to understand where is the fourth lake in Sinnoh. Please help me 

[Yverneas]

Hello @Bobby2,

 

I'm happy to see that you are attempting to complete the quest on your own.  

I'll also make an effort to avoid giving away the location, although I will provide a small indication. "It's the same place where you get teleported back from Distortion World after defeating Giratina and talk with Cynthia while doing Sinnoh's story." 
If you are still not remembering the place, just open this spoiler. 

 

Spoiler

Sendoff Spring - East to Route 214

You will also need a Pokemon who knows Rock Climb or Ansel Rope and a strong team with max happiness Garchomp to fight a Boss. 

 

Hope this will help you. If you have any more questions regarding this then feel free to ask them here. 

 

Kind regards,

Yverneas.