Have had constant tension in head/neck/face for 4 years. around ears, sides of neck, behind jaw, and under chin. have anxiety, could this be related?

Muscle tension: Certainly, your muscle tension may be related to your anxiety. Consider trying progressive relaxation using an audio-guided app or CD. Some are free. Other treatment options might include massage therapy, physical therapy, counseling to find out why you have so much stress and help you deal with it. Just a thought, but have you checked with your dentist about possible TMJ dysfunction?

Chicken or egg?: It's hard to say which came first (as in the chicken or the story): did your anxiety cause your tension, or did your tension cause your anxiety. Head and neck muscle tension can certainly cause pain which can make you anxious. Talk to your doctor about physical therapy and/or exercises to help with the muscle tension. Controlling that may control your anxiety better.