Engine Fuse Box

Well while I was replacing my altenenator today I took a peek into my fuse box in the engine compartment and found the regular 15A fuse for the 4X4 basically missing I looked at it futher and think it may have melted? I know the 4wd is working though because I had the car on a lift a few days ago and all 4 go fine. So my question is should there be a fuse there? I never looked at it before so i am unsure and if there should I was wondering why it being burnt up and all it is not affecting anything?