The good news is that it is not of immediate concern to your doctor. That they are following it rather than reacting to it now means they view it as likely harmless and not cancer related. If you are concerned they are not being proactive enough, you could go for a second opinion to a second doctor. In reading, cancer is less likely due to the location of right middle lobe. 4mm is very small as well. And for some people, these nodules can actually come and go.
The wait and see approach is very typical for spotting small nodules like this and here is some information on that. https://www.medscape.com/viewarticle/805296
Please let me know what you think of this information.