I was diagnosed with panic attacks in December of 2011 and prescribed clonazepam, which I haven't taken in 4 years. Ok to take now? I'm having trouble

Panic attacks can. be very unpleasant but are very treatable. See your doc for a new prescription if he/she thinks its appropriate. More to the point, learn some skills to deal w/panic attacks from a clinical psychologist. Remember that panic attacks are self limiting. How is your diet and exercise routine? Peace and good health.
Panic attacks & meds. Any of the benzodiazepines can lead to development of tolerance, but used in moderation they can be very helpful. It is ALWAYS a bad thing to end up in an ER in a panic attack--better to use the med as your doctor prescribed, PRN usually, and find an expert trauma/anxiety therapist. Strongly recommend EMDR Therapy, with CBT skills also for safety and grounding.
See your PCP. The pills are 4 years old and may not be as potent as they were so it is best to go back to your doctor and get reevaluated.
Anxiety. I commend you for not taking Clonazepam for 4 years. I understand your discomfort and your desire to have relief. After doing well for 4years without medication, it is important to see a Psychiatrist for Diagnosis of Anxiety, panic symptoms or Panic Disorder. You will benefit from Psychotherapy, Breathing Relaxation, Visual Imagery, Tai Chi, Yoga, and Meditation which will help with stress.