Tropical Depression 4: 5-day forecast track. NHC/NOAA graphic

The fourth tropical depression of the Atlantic hurricane season has top sustained winds of about 35 miles per hour -- just four miles per hour under the threshold for a named tropical storm. It was about 210 miles west of the southernmost Cape Verde Islands as of 5 p.m. EDT, the National Hurricane Center said.

"If it becomes a little better organized in the next six hours, it could become a tropical storm," said Lieutenant Dave Roberts, the US Navy hurricane forecaster at the center.

The system, moving west-northwest at about 18 mph, isn't expected to be a threat to land for the rest of the week, Roberts said. The storm will still be "well east" of Bermuda on Sunday, he said. Current indications suggest it could turn to the northwest on Friday or Saturday, diminishing any threat to the US, Roberts said.

The system is forecast to reach tropical storm status on Tuesday night or Wednesday, meaning winds of at least 39 mph, the hurricane center said. It would become a hurricane if winds reach 74 mph.

There have been three named tropical storms since the Atlantic season began June 1, but no hurricanes.