NOC system is created for the job market in Canada, so you need to understand the difference between NOC 4011 & NOC 4021 from that perspective.
In Canada, colleges & universities are very different -- the former give diplomas/certificates and are usually shorter than 3 years, while the latter offer undergraduate degrees and usually require 3~4 years to complete. You can see this
link for more details.
That being said, this Canadian system of college/university might not exist in your country. In USA, there is often no clear distinction between these two. For example, although Dartmouth College is called college, it is essentially a university because it provides undergrad degrees.
In my opinion, since the college you teach offers undergrad degrees, you probably want to pick NOC 4011. It has nothing to do with whether the college is affiliated to a university or not.