Well, that is the expected behaviour. As described in the documentation, the rubric grades are normalized first. So if the reviewer gives the grade 4 from a scale 4 to 8, they actually declare the worst possible performance considered. Which maps to grade 0 in Moodle.
I can see you point though and this is how I would try to achieve that. Set the Grade for submission in workshop to 100 (not 8). Edit the rubric so that it contains three criteria with level grades 4, 5, 6, 7 and 8. The final grade for submission will be a number from 0 to 100 (aggregated as a sum of level grades and mapped from 4-to-8 range to 0-to-100 range). Then go into the Gradebook and try to play with the display format of the grade. There is a way how to display grade values from 0-100 as a letter grade from scale 4 to 8 (which is what you want).
Also note - if you want the criteria grades being aggregated using the average (not sum), you might want to use the Accumulative grading strategy.