iOS8 Health: 'Motion & Fitness' privacy setting not found

My partner has an iPhone 4S (MD239B/A) and I have a 5S (ME434B/A), both running iOS 8.4.

The Health app is not recording motion activity on the 4S but works fine on the 5S.

iOS says that 'Motion & Fitness' should be tapped under Privacy settings to authorise the phone to collect motion data. This setting is not shown on the 4S Privacy screen. Two screenshots are attached, first the 4S then the 5S.

The Apple website (foot of linked page, small print) says that the Health app works from iPhone 4S onwards. This website confirms the model number of our 4S, MD239B/A, as a 4S.

Please could someone advise what we need to do to enable 'Motion & Fitness' on the 4S?

Thanks.