A fathom is a unit of measure often times used in measuring water depth. 1 Fathom equals 6 feet (~2 meters). 4 fathoms is how far over your head the water was at these coordinates during Hurricane Harvey. The "x4" suffix applies due to this particular location being 4 fathoms under 4 times since 2017. Fathom that! 
As a bonus, in case you were wondering, the "structures" the cache is hidden within were placed here decades ago by the U.S. Army Corps of Engineers. Their purpose is to prevent erosion of the river bank during high water. Note how some are barely peeking through the surface and how they are all cabled together.