江、河、淮、濟也。
It refers to the Jiang, the He, the Huai, and the Ji.
四瀆何以視諸侯？
Why are these rivers compared to the regional lords?
能蕩滌垢濁焉，能通百川於海焉，能出雲雨千里焉。
It is because they are able to wash away mud and filth, and are able to provide pathways for the hundred streams to reach the ocean, and are able to give rise to clouds and rain extending a thousand li.
為施甚大，故視諸侯也。
Because the favors they extend are very great, they are compared to the regional lords.
Viscounts and Barons As Imaged in Hills and Streams