I had radiation four years ago for prostate cancer, now my seman is bloody. is this a medical emergency?

No: Not an "emergency" per se. You should however let your urologist know about this. Most of the time hematospermia is not significant. You may need cystoscopy or antibiotics.

No but get checked : Blood in semen, while alarming, is uncommonly a sign of severe problems. You can see a urologist (i am in mount kisco) and get checked out just to be sure.