I sympathise with you - but from the other side. Having suffered with anxiety and depression in the past, I know how you get into a mindset that you can't do things, it's all not worth it - and you have to want to get better, to lift yourself out of it, to make life worth living again.
What treatment has he had, is he still under the GP, has he seen a counsellor or therapist? Four years is a long time, but it's not an impossible time - I was under the burden for longer than that, but I did eventually seek help, and hypnotherapy helped. It's a long slow haul back up, and I know I wasn't easy to live with during that time. It's tedious and frustrating for the partner but to the sufferer, it's as real as having a bad leg, and you can't see a way out of it. Nagging, I'm sorry to say, doesn't help - encouragement helps.
You're not wrong in the way you feel, but he has to want to get better and if he was like me, these things seem insurmountable. He needs professional help before this goes on any longer, or it may never change. IMO.