Sometimes you can't, quotas aren't a useful solution. Check your recruiting approach. Are you recruiting from male dominated events? Are there more balanced forums or even female dominated forums you can also recruit from but haven't yet? Do you read the name of all applicants with their resumes? Try sorting applications without knowing the name, gender, race and see if perhaps you had an unintended bias in how you filtered them. Check your behavior. Do your female engineers end up as project tech writers and configuration managers more often than the men? Is that where they want to be or did you just happen to put them there? If so, why? Check the behavior of your staff, is there a guy who keeps hitting on the female staff or a particular person? I've seen this happen and several women leave before management acknowledged that his behavior really was a problem.

In the end, your 4:1 ratio may be fine, you may have hired the appropriate people for the position and there's no reason to fret. Same with ratios for other demographics. But recognizing that intended or unintended biases can be at play is important if we want to change STEM at large.