WASHINGTON—Borrowing by Americans fell in January for the first time in five months, as the use of credit cards fell to the lowest level in four years, offsetting gains in auto loans and student loans. The Federal Reserve reported Friday that consumer borrowing fell by $1.3 billion in January, the first setback since a $9 billion decline in August. The weakness came from a $9.9 billion decline in borrowing in the category that covers credit cards. It marked the fourth straight decline in that category and was the biggest drop since a $10.8 billion fall in August. It pushed credit card activity down to the lowest level since January 2017. The category that covers auto and student loans posted an $8.6 billion increase in the first month of 2021, following an even bigger gain of $11.6 billion in December. Consumer borrowing is closely watched for indications about Americans’ willingness to …