Hi and welcome to the board. We don't usually handle v4 support as Ritlabs official support does that - we're a legacy facility, but I just checked and the same thing happens with v3 of TB! so that puts it in our ball-court too.

So this is how to fix it - at least in v3.6 which I'm using:
1. Create your new inbox-known folder by right-clicking on the Account Name and select New > Folder and call it $Known$
The new folder appears at the bottom of the list of folders for that account below Trash. It will not have an arrow icon.
2. Click on the new inbox-known folder with your mouse to highlight / focus on it, and then on the keyboard press and hold the SHIFT & CTRL keys and press the UP arrow key, this will move the inbox-known folder above the Trash folder. As soon as it is top-side of Trash the arrow magically appears. Keep pressing the UP key until the folder is where you want it to be in the list.
Let us know if this works for you with v4.
