Hi Kate,
Do you have individual emails, programs, or both in your nurutre stream?
If a lead is already a member of a program, and that program is in your Nurture Stream,
that lead will be skipped when the program is cast by your Nuture Stream.
You could create a smart campaign that looks for leads who download/downloaded the FAQ document in Email 2, with a Flow action to add them to a program in your Nuture Stream that contains Email 4. In that scenario, only people in the Engagement Program who
didn't download the FAQ document would get Email 4.
Hope that helps!