I've just checked my friends configuration and he is using the Paypal Express Checkout from FoxNetSoft, would this solution likely expose the Pay in 4 option if configured in Paypal?
I think in most cases, any PayPal "buy now pay later" option is dependent on what PayPal shows the customer. You may be able to control that in your PayPal account settings, but keep in mind that you (the merchant) always gets paid in full immediately. It's just that the customer pays PayPal in installments. If you want to show a "Pay in 4" button in your checkout page, then the nopCommerce PayPal plugin must be a "Button Payment" type of payment plugin. (You can discuss that with the vendor)