Two Solutions:
1)
AnyMotion = Motion3 || Motion4 (With Repeats Checked)
NoMotionFor15 (AnyMotion; NOW > 15:00)
[hr]
2)
NoMotionFor15 (Motion3; NOW > 15:00) AND (Motion4; NOW > 15:00)
[hr]
What you had says
that if Motion3 is followed by Motion4, then the next evaluation of NOW that is 15 Minutes after Motion3 will cause it to be true!
A semi-colon separated sequence expression is true when:
All of the conditions are in the specified ORDER
And the Delta time from the first even to the last event meets the specifications (if provided).