Power = v x v / R
If voltage goes from 240 to 208 resulting output should be 75% of 4500 or 3375 to 3380 depending upon how the number is rounded.
The above equation is true for resistance loads which are linear. If you cut voltage in half, current will be one half. I have seen two water heaters which were advertised to have “self cleaning elements.” These elements did not have linear resistance. When voltage was 120, current was about 75 to 80% of current when voltage was 240 volts.