I'm four months late. Can I be pregnant with a negative blood test?

Need sonogram. Remote possibility you need to rule out other causes. You may need a sonogram as the first step.
No. If your pregnancy test is negative by four months after you missed your period, then you are not pregnant. Unless you conceived in the last week or so. A pregnancy test is positive by about ten days after conception, which could be about four days before you miss your period. After four months of no periods, you should see your doctor.