I'm currently using 4.25 and 800 DPI on Overwatch with a 103 fov.

I would like to have the same sens in csgo, but I can't really figure out what I have to use exactly,

I'm playing on 1280 x 960 4:3, my question would be what do I have to fill in exactly to get the same sens

Since Overwatch and CS:GO have slightly different FOVs they can't share the same 360 distance so you have to use a converter. The best one in the calculator so far is Viewspeed v2. For Overwatch to CS:GO you input your sensitivity, DPI and WPS, resolution and FOV in the top section and then unless you change your DPI or resolution when you play CS:GO all you need to do is select Viewspeed v2 in the "Convert to" dropdown and select whether or not you have Raw Input enabled in CS:GO you will get your CS:GO sensitivity in the CALCULATIONS section below the INPUT section. The FOV Type for the games is Hdeg Res and Hdeg 4:3 respectively so make sure they are correct as well.

The sensitivity I calculated for you in CS:GO is "1.115126"