Well I have been using GPS's now for 20yrs, I still have an use my Garmin Etrex, I have only ever seen 4mtr accuracy, since it only uses the US "military sats" and found out it was SA "selective availability" and the Glossnos Sats are Russian, so do you think they are more accurate ???. If your a professional Surveyor your equipment is offset for SA and works to mm acc. I have taken info from Topo/Geological map systems for years either from paper or digital maps and walked to an abandon mine and when been within 4mtrs of the hole it says "arriving destination" so I don't see why you are so worried about accuracy, am not going to send a missile through your window huh.
This the way.