Is there any real difference between "fòk ou...", "ou dwe...", "ou fèt pou...", and "piga ou...", other than the last one is negative?

No, not much difference.
They're all about obligation, necessity, duty.

Pinga ou is also about giving a warning or advice.

Examples:

You must go talk to her. (can be accurately translated many ways in Creole)

Fòk ou al pale avè l.

Fò w al pale avè l.

Ou dwe al pale avè l.

Se pou w al pale avè l.

Ou fèt pou w al pale avè l.

Ou oblije al pale avè l.

Pinga ou pale ak moun ou pa konnen.

Don't talk to strangers.

Pinga w al dòmi san priye.

Don't go to bed without praying.

Pinga ou bliye m.

Don't you forget me.

Pinga ou ouvè bwat sa a.

Do not open this box.

Ask me anything