Agree with Boondoggle that they can begin spawning at 2-1/2 to 3".

I think a well-fed GSH at a fish supplier can grow to about 4" in one year. However, there is a pond owner that got 5-6" long GSH brooders in April, so either they can grow even faster under optimal care, or the supplier slowed them down and the ones that size were at least 1-1/2 years old?

How large are these shiners? If you bought 4" shiners in the spring, I would be pretty confident they were about one year old, and should be ready to spawn for at least one season in your pond.

[Non-expert opinion, just based on what I have learned on Pond Boss.]