Dan Code

I would like to add a city size requirement for building pioneers as I am somewhat of a perfectionist and don't create a lot of cities right off the bat. Suffice to say, I typically don't have a lot of territory. I would like to make a lvl 4 city a requirement for building pioneers. I think this is justified as settlements will work toward their own survival and establish themselves before creating more settlements. So basically, how can I create a requirement for a unit?