My guess is your EC meter does not go to 2 decimal places. PPM is just EC converted.
Just to clarify you have an EC of 0.04 and a PPM(500 scale) of 20.
No you do not need an RO system. My RDWC grows have all been with tap water of 180ppm(500 scale) EC 0.36.
I've never heard of that and I've never had any problems, in my opinion, you can use it without worry, check the pH, feed properly, and you'll do great.