On zoloft (sertraline) (175mg) and klonopin (0.5mg/day) for 4 months, still life-impeding anxiety. how safe to stay on klonopin longer term since ssri is not worki?

Safe: Your dose of clonazepam is low enough that it does not generate any particular concerns, and in fact is surprising that it that helpful. You plan is sound: continue it until you find another medicine at sufficient dose, long enough, that you don't need the clonazepam.

Very dangerous: benzodiazepine like Xanax (alprazolam) and Klonopin are very addictive and has the potential to cause physical and psychological dependence. May be useful in short term ONLY Please see a psychiatrist for non-addictive alternative for your anxiety