MG/L = ppm (almost exactly).
If you want to be exact.....4.0 mg/L = 4.004 ppm.....
So yes .... it looks like you have very soft water
I find my water profile varies throughout the year.
I bought a GH and KH test kit for aquarium water testing at pet store.
I usually do a KH test (which is alkalinity) just to double check the alkalinity of water the day I brew. It is not exact but it is just a way to double check it to make sure in right ballpark.