What is Pay in 4?
Pay in 4 eligibility will vary by merchant and individual shopping experience. The Pay in 4 option will be displayed in PayPal checkout if the option is available for the specific merchant as well as the transaction amount and type. Keep in mind, not all goods and services are eligible. If for some reason the items in your cart are not eligible for Pay in 4, the offer will not be available.
Why was my Pay in 4 approval not used for my completed purchase? 
Your Pay in 4 approval is valid for that specific shopping session in which it was granted. If you change the payment method during checkout and don’t switch back to Pay in 4, your purchase will be charged to that changed payment method. In addition, if you experience a disruption in your session (internet glitch or session timeout) the approval may become invalid. In that case, you would need to apply again to use Pay in 4 for a transaction.