Does a low C4 level (10) always indicate a problem (such as myeloma, MGUS, auto immune)? Can it be just an insignificant test finding?

No. First you'll need to tell us why the test was obtained. If you have frequent swelling, abdominal or throat obstruction, this finding may indicate a condition known as hereditary angioedema. However if you are otherwise healthy, just follow-up would be sufficient. We usually treat the disease and not a lab finding.