Hi katch,
Gosh. Four years is a long time, and I can imagine how frustrating this must be for you.
Unfortunately I can't offer specific medical facts beyond what your doctors are able to see, however I have also been with someone going through very tough times with their health for many years. It can be quite a roller coaster.
Regardless of the diagnosis, the most important, and most supportive action that you can provide him on your part is the sense of normalcy. I remember that one of the most frustrating things for my girlfriend was the feeling that people would treat her differently or pity her. She really resented this pity as it only re-inforced for her the sense that something was wrong for her. She just wanted to live a normal life, and to be able to enjoy the little things just like everyone else.
The other thing that is very frustrating is the lack of standard practice in medicine, and this would also drive her nuts. One doc would say one thing, the other another. Don't give up, and keep looking for answers. I hope that your doctors will be able to track things down.
We never know what life has in store for us, even if we are perfectly healthy, so the absolute best thing that you can do for him is to uplift his spirit. By your previous replies I can tell that you genuinely love your husband and put forth everything you have for him.
Make sure to find time to decompress from this pressure and to breathe peace into your own life as well as the relationship.
I hope that everything will turn out for the best. Please let us know how things develop.
Sincerely,
Daniel