The number of three to four inch gainers is small. But the number of guys who wish to gain three or four inches is small also.
All you have to do is research the goals of the PEers on this board. I don’t think you can find any with an ultimate goal of ten inches when starting at six inches. I believe the average length goal is eight, from a starting average of six.
The only guys I have seen that mention a goal of ten plus are starting at eight plus.