You shouldn't use an iPod Touch case designed for the second and third generations on a fourth generation iPod Touch. The fourth generation iPod Touch has a camera and microphone, so the third generation case would cover both of those up, and would not fit it anyway.
No. The 4th generation iPod Touch and the iPhone 4s have basically the same length and width, but the iPod is 7.2mm thick while the iPhone 4s is 9.4 mm. They also have slightly different shapes. Technically, since the iPod is smaller, it would probably fit, but it wouldn't stay on or work properly.
1.) Get a guitar pick. 2.) Stick the pick in the side of the case. 3.) Swipe the pick slowly through the side of the case. 4.) The case should now be loose. 5.) Remove the ipod from the case. 6.) Enjoy!