Personally I don't think it matters that your fourth son doesn't have a bible name. I agree with your husband it should be a name that your both like.
If you still wanted to use biblical names though here are some that would go with your sibset:
It sounds like you have already given consideration to other biblical options. Don't sweat it. It's much more important that you both love the name you choose.
I love the Micah suggestion. You could also consider Asher, Luke, and Jesse.
I think you should have another Biblical name for the fourth but don't worry about it. If you can't find a Biblical name you both love then don't worry about it, just go with another name. I really like Joshua, Noah, Levi and Caleb together.
These are some of my favourite Biblical names -
Enos - Ee-noss
I don't think it matters at all as long as you and your partner like the names :)