Feist performed an alternate version of "1234" on Sesame Street during its 39th season (2008), teaching children to count to the number four. She said working with the Muppets was a career highlight.

Based on this passage, who sings the 4 song on sesame street? Let's give stream of consciousness first:
To answer the question, consider the following: Feist performed an alternate version of "1234" on Sesame Street during its 39th season (2008), teaching children to count to the number four.
So, the final answer is Feist.