The microSiemen is the unit of electrical conductance.
It should be shown not as uS but as the Greek letter "mu" immediately preceding the letter "S", but I can't find the "mu" on my keyboard, it's in the character map, I think.
A uS is, I think, equivalent to 1 part per million of total dissolved solids, so 4 uS is 4ppm TDS.
Hope this helps.
Baldeagle in Staffordshire