OK, label matches previous numbers.
On a hot day, more current leaks back through the PV cell. But I think that affects power output more than Voc.
Have to wonder if inverter does want higher voltage, since you're near lower end of MPPT range.
Also, if there is a bad connection.
Could try boosting voltage somehow. If not a 5th PV panel, then a 12V car battery in series with panels, raising string to 152V.
Do you have an ammeter good for 10A? Cover PV panels, disconnect from inverter, connect ammeter between PV+ and PV- (that shorts out the PV string). Uncover panels and read meter. Check for any connections getting hot. With 1kW, it could get hot fast and melt a bad connection. Doing this first while sun is off-angle would reduce current and heating.