My daughter has done these before at school and I know I've seen them all over blogland so this is definately not an original but still fun nevertheless.
Start by coloring coffee filter with washable markers.
Spray coffee filter with water and the colors will mute and blend. (Note - cleaning out sink beforehand is entirely optional.)
Let dry on a rack with a piece of toweling underneath.
Hang with a string in the window.
Reminder - when your 4 YO asks you if she can do the spraying part all by herself. Remember to tell her she has to wait for you.
You can also fold them accordian style, attach in the middle with a pipe cleaner and spread the wings to make a pretty butterfly!