Are "supplemental" and "Part 4" the same thing?

YES!  When the online application was first created the program specific questions were known as "Part 4" of the application, with the first three parts being Institute wide.

We no longer officially call them "Part 4" since the application has been broken down into multiple sections.  It is now called a "supplemental" meaning the program specific questions that "supplement" the Institute questions.

Depending on who you talk to and what you read, you may still see either of these terms mentioned.  They refer to the same thing, the program specific set of questions.