Instead of adding more detailing to Hulk and Iron Man, you will first need to start sketching out Thor, and Captain America. For Thor you will need to sketch out the muscular arms, and hands, and for Captain America you will sketch out the face, shoulders, and back. Now you can focus on getting Iron Man completed. You can do this by adding the detailing and definition to his head and or mask, draw out the right arm and hand, and then completely draw out the left hand and be sure to include the circle in the middle and on the chest.