Bonding: impossible to create

RouterOS Beginner Basics
1

Hi,
I tried to make a bonding on a Mikrotik CAS125-246-15-2HnD-IN with a WinBox v6.28 but it was not possible and I got an error message (see snapshot).
What shall I do?
Thank you.

2

Seems that ether4 has already ether3 set as master port so WinBox complains.

3

When I set port 3 as master port the error message is similar. There must be another Problem.

4

They both should have no master port set as they will turn into slaves of BOND interface.