Hi Pari,
What you’re describing sounds like PayPal’s Installment Plan feature, which s2Member does not currently support.
To make your scenario work with s2Member, you’ll need to set up a regular recurring subscription and then present an upgrade option to your members after 4 months. The upgrade form would be a Billing Modification Form (Dashboard -› s2Member® -› PayPal® Pro Forms -› Billing Modification Forms) set to charge for 1 month for unlimited lifetime access (so you’d charge for 4 months, present the upgrade form, the member would pay for the 5th month and then automatically get unlimited access and not be charged anymore).
The Billing Modification Form would cancel the recurring subscription, charge for 1 month, then stop charging, and the member would be given unlimited, lifetime access.