Per the IRS Direct Deposit Limits webpage:

In an effort to combat fraud and identity theft, new IRS procedures effective January 2015 will limit the number of refunds electronically deposited into a single financial account or pre-paid debit card to three.

The fourth and subsequent refunds automatically will convert to a paper refund check and be mailed to the taxpayer.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.