You need to make a new matte in the Titler or Photoshop. Each black bar should be 132 pixels in height.
Here's a formula for 'letterboxing' any footage into any desired aspect ratio.
1. Do the math on the ratio for the footage you have, in this case 16 divided by 9 = 1.7777
2. Take that ratio and divide it by the ratio you want, in this case 2.35*, which comes out to be .7565
3. Multiply that number by 100 for a percentage, in this case it rounds out to 76%.
4. Subtract that percentage from 100%, you get 24%.
5. Divide that percentage in half, here it would be 12%.
6. Apply a Top and Bottom Crop using that percentage.
*On a side note, the theatrical standard has actually been 2.39:1 since the early 70s, even though people often still call it 2.35.
That's a really convoluted way to do that.
This is easier:
Take the DAR (display aspect ratio) and divide it by 2.35
4096 / 2.35 = 1742.
So actual visible area would be 4096x1742.Then minus 1742 from 2160 (height of 4k) to get what is left of black pixels.Divide that number by 2 to get the top and bottom numbers evenly.