The lag that is downloaded under the lower edge of the rear window of Citroen C4 is not recommended to be removed by rag, brush or finger-it may damage the paint of the car. To remove the moisture between the lower edge of the back glass and the door of the fifth door, you can use the compressed air purging service offered by most of the car mods.
You can also solve the problem radically: process the lower edge of the back glass with silicone grease or smear a transparent silicon sealant. These actions will help to prevent the appearance of moisture and dirt under the rear window of the vehicle.