I'm not a lawyer, but based on my understanding you can use CKEditor 4 with its LGPL license (as long as you don't modify it).
I am starting to get a small (just me at the moment) web design company off the ground and there is one thing that is a bit fuzzy to me - whether I can legally use open source apps in sites that I ...
The example is based on CKEditor 5 but it shouldn't be difficult to modify it to use the previous version, which has a less restrictive license.