Cups made of paper
Craft scissor (with extention)*
Take a cup and measure 4 cm in the height and mark. Then cut in that height with the craft scissor. Choose any extention you want. Those scissors remember me to the ones from the Kindergarden ;)
You can also put the cups in the oven in case you don’t have a muffin pan. That way the muffins will keep the typical muffin form.