Should I do a Master's degree if I am already employed in the field?
First, research if your company has incentive programs for higher education. Many top companies offer partial/full tuition reimbursement to qualifying employees to pursue a masters. If this is the case, use the program to take 1 or two classes a semester. Frankly, a person with a bachelors can usually attain the same titles and salaries as a masters. The biggest difference in salaries/titles/responsibilities are between masters and PhDs. The main difference is that a PhD can be a PI on research grants/studies.
I went back to University when I was 30. Sometimes the market calls for a change in careers. But as I stated before, you may not have to "return". If your company is flexible, you could attain a masters on the side within 3-4 years by doing classes in the evening/weekend/online.