PayPal today introduced a new installment credit option for PayPal users called “Pay in 4.” The name itself explains what the service offers — basically, it’s the ability for customers to pay for purchases, interest-free, over four separate payments. The service is an expansion on PayPal’s existing lineup of Pay Later solutions, which also includes PayPal Credit’s revolving credit line and its Easy Payments. With Pay in 4, customers can choose to pay for purchases between $30 and $600 over a six-week period. Because it’s included with the merchant’s existing…