Another nice to have when upping the character limits, is also changing it for the /api/v1/instance endpoint, which apps like Tusky parse to do it in their gui.

It's a bit more involved since Mastodon doesn't send this key at all, so you need to add it to the app/serializers/rest/instance_serializer.rb

Add the attribute in the list of attributes, as :max_toot_chars
Then define it next to the other 'def' blocks like

def max_toot_chars
4096
end

Restart mastodon, and Tusky should pick up this change when opening the compose menu :)