Are you wanting to add the PayPal buy button on your product pages? Or are you meaning that you'd like to add the PayPal Buy Now Pay Later (or Pay in 4) icon on the footer of your store?
If you're wanting to add the PayPal dynamic checkout button on your product pages, depending on your theme you can enable this within your Theme Editor, as mentioned on our Help Centre document here. Once enabled, the dynamic checkout button that's displayed on your product pages depends on a couple of factors. This includes:
Because of the factors mentioned above, customers might not see the PayPal button on your product pages, but can instead see other branded buttons, such as Google Pay for example. You can read more about how the dynamic checkout button operates here.
If you're wanting to add the PayPal icon onto the footer of your store however, most themes would normally enable payment icons to show up on your storefront's footer automatically. If it's not showing on your store, you can check your Theme, as mentioned on this Help Centre document, to see if you've enabled the payment icons to appear on your store, or add it in manually via coding.
Looking at PayPal's website here, for their verified logos and icons, they don't look to have a specific Pay in 4, or Buy Now Pay Later, icon. You can however check with PayPal themselves too, to see if they have this available for you to add to your store if that's what you're after.
For more context, the reason why PayPal might not have an additional Pay in 4, or a Buy Now Pay Later, icon is because this method is offered within the existing PayPal gateway. If you already have PayPal integrated on your store, you don't need to do anything else to enable this. When a customer selects to use their PayPal account to pay at checkout, they are redirected to their PayPal wallet, where Pay in 4 will be offered as a payment option if it's available.