Can hcg levels 4.2miu/ml show up 28 days after sex on blood test? does every woman have hcg levels regardless of pregnancy or not?

Less than 5: An HCG value of less than 5 28 days after sex essentially rules out pregnancy. Normal levels in non-pregnant state are less than 5. A level of greater than 25 is diagnostic of pregnancy. Between 5 and 25, the test needs to be repeated in a week or so. As I mentioned earlier, from the information you provided your highly unlikely to be pregnant.