Just tried to post a new post of 4171 characters, the animated gif indicating it was posting would just spin endlessly before throwing an error, "Whoops, something went wrong. Please try again later." When I trimmed it under 4000 characters, 3791 to be exact, the error went away.

11 Posts (+1), 50 Cmts (+39)
You're correct, there is a max length on post text of 4000.
text: STRING(4000),Was this a real use case, or were you just trying to see how long you could make it? I could increase the max length, but it would come with a performance hit.
Either way, I definitely need to improve the error handling. Good catch!
10 Posts (+141), 139 Cmts (+107)
It was a real use case. Was posting a Love Report that in full is probably 6000 characters. Maybe you could up to 10k? Not sure how much that might impact performance of course...