Hi Jiri,
If you apply 4kV to the supply, you will need extra protection for U20 as it is limited to 3kV isolation.
You may find that you will have better success if AGND and DGND have a low impedance connection under the ADS1217. You might consider making this a solid ground plane area. Some of the analog return paths crosses ground splits. You need to make sure that all return paths are as optimal as possible and no analog signals cross out of the analog plane.
The connections to R32 and R42 in the thin board area will act like antenna.
Best regards,
Bob B