Sketching in MN4 Mindmap Space

Hi,
I was able to sketch in MN3 Mindmap space, which make it really easy to draw out diagrams in class. But in MN4, I could only draw in the document, and the pen tool in Mindmap became just a purple highlighter. Am I missing something? How do I sketch in MN4’s Mindmap?

MN3:

MN4: