Well, I like mine as well, but they do not all work as well as yours. I have heard from many who did not find success.
As for the accuracy of yours, you are possibly one of the lucky ones.
Getting a unit bench tested is a good idea, but for most people, that can double the cost or even more.
an accuracy of +/- 0.5 pct is 3.915 +/- 0.0196 Hz or 3.894 Hz to 3.926 Hz
If yours is 4.0 +/- 0.5 pct, then you may not be getting as good of results as you might otherwise. Or is 4.0 Hz actually better than 3.915? Or is each better for some things than the other frequency?