Hi,
One thing I would really like to see in both the x sheets and the timeline is an element you can actually draw in and make notes in. In the OLD days, 
I would indicate hits by drawing curves and notes in a column of the exposure sheet or on a bar sheet (timeline) to plan and time my animation. It is a very helpful technique and would be very useful in toonboom.
V4 looks pretty clean, I just upgraded yesterday… I waited a bit unlike the upgrade to V3 which I ordered in advance at the discount price and then could not use for 2 months because of a tablet bug. Must admit I feel kinda bad not having taken advantage of the pre release deal on V4, but V3 was such a bummer I figured I’d just wait for the bugs to settle this time. It looks really good so far… thanks for the great work.
Here are two useful tips that might address your request at least until something better comes along. You can add “cell notes” to any frame which are cell-frame specific. (IE cell 1 at frame 10 can have a totally different note from cell 1 used at frame 24 etc. ) Secondly, create a “notes” drawing element and make your notes in that element but disable the element in your timeline so it isn’t rendered. Now you can see your notes through the auto light table feature and you can also see them when you view thumbnails in the x-sheet and you can even see them when you select a cell and look in the cell swapping panel’s preview window. Not as slick as your suggestion but very effective for keeping running notes ect. on your x-sheet. -JK