Hi, Graham
The “Wait up to” here for the if/ then means that the data that meet the condition of waiting until a certain time period go to the Yes branch, and those that do not go to the No branch.
In your case, if you receive the first email within four days, you will be sent the second email, and if you don't receive it, you will go No branch after four days.
So all you need to do is add a “Wait for” before the “if/ then” and set the “Wait for” time to 4 days.

If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya