In the opinion of your seller, you paid too much for shipping. So she refunded you.
This is a good thing and nothing to be suspicious about.
It would be better not to try to remove the $4 from PP to your bank account since PPwill charge a fee for any transfer under $150.
Just use the balance on your next purchase.
Don't overthnk it. The minimum wage in Ontario if 17 cents a minute. That means $4 is worth no more than 23 minutes of your time. Less if you earn a living wage.