The change from 8 to 4 and back again is only discernible in ours from the change in exhaust tone.
With the roof up and cruising I wouldn’t even know unless I noticed the dash display change from 8 to 4.
The vacuum sensor on the brake booster could be the culprit. It’s a cheap and easy fix if it is. If it is defective there will be a trouble code stored for it. Have you had the car checked for codes at all? Should really be the first thing you do before you start throwing parts at it although the vacuum sensor is so cheap it’s worth doing first if you’re going to have to pay for a diagnostic which will generally be more expensive.
Also have you checked and confirmed your exhaust flaps are working correctly? Stored codes for a failed flap actuator might have disabled the ECO system too.