Looks perfectly healthy. Some plants just grow quicker than others. Doesn't seem to be any indicator of quality though. I've had plants that are smaller than that at 4 weeks and turned out to be killers, and my current indica hybrid is massive at 22 days, and might turn out to stretch by the looks of it. Some plants will shoot up very quick third, fourth or fifth week.
The pics attached Is a 21 day fat monkey auto. Have heavily tied it down as it seems to be stretching, but would be around 12/14", and onto 4th node. It was tiny 8 days ago in the smaller pic, but over 8 days it has gone insane.
You'll probably find it will take off very soon.