I updated a project to SS4.2 today with composer update and it was a bit of a mess. I got some TinyMCE errors right away, but can’t say now if they were the same as you’re getting (especially since you reported having this issue on SS4.0?).
SS CMS ended up at v4.2.1 and SS Framework at v4.1.2. Seemed to be to do with the new ‘recipe’ approach. At some point I’d apparently inlined my requirements and I ended up with a weird soup of incompatible dependencies despited Composer not reporting any problems.
Anyway, I cleared all of the 'silverstripe/' requirements from my composer.json and replaced them with just "silverstripe/recipe-cms": "^4.2.1". Did a composer update and now everything is fine again.