I apologize for the informality in the previous response. Here's a more professional response to your query:
Within Poland's extensive hydrological network, two prominent rivers stand out for their concise four-letter designations:
Wisła (Vistula): Holding the distinction of being Poland's longest and largest river, the Wisła traverses 647 kilometers (402 mi) entirely within the nation's borders.
Warta: This significant tributary of the Oder River stretches for 808 kilometers (502 mi), solidifying its position as the Oder's second-longest tributary.