Hi,Your husband needs to see a doctor,and be checked.It could be the start of parkinsons syndrome.However it could be many other things or could be nothing at all.But if he has not done this most of his life and just started 4 years ago,then I would get it checked.Good luck.
I am shaking my leg right now as I type. If this is what he is doing (shaking a limb) it could just be a nervous habbit that he is completly unaware of.
If it is more of a tic like movement then he should probably see a doctor for a referal to a neurologist.