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This should be easy enough through the use of regular group permissions and the subscription module.
You should be able to configure wf-sections to allow registered users to view category 1, 2, and 3 (non-paid) categories. Create a new group (paid users...or something like that). Assign permissions in wf-sections so that the paid users group has access the non-paid categories, and also category 4 (paid category).
Then set up the subscription module with a subscription that will add a user to the paid users group.
This is similar to how the subscription module creator has set up their very own website
here. For example, a registered user has access to certain sections of the website and can download the subscription module, but to download the payment processing addons, you must subscribe to be added to the appropriate group with permissions to access the payment processing downloads.
