It all depends on what you want the knife to do, and where you live.
Let me say this right now, at the risk of getting lynched:
THERE IS NO ONE TOOL THAT CAN DO IT ALL WITH EASE
If you are going to pick one tool, pick a big one, because a big tool can do what a small one can't, the reverse is not true. However, there are things that the big tool is a big pain to do. Which is why, throughout history, people carried at least two.
Provided you carry a large tool (axe/hatchet, machete, kukri, tomahawk, etc), then a 4 inch knife can do all the rest.
Now analyze what you are really doing. Are you just going for a hike? You probably don't need any knife.
Are you going to build some natural shelters? A knife is great to have, and you may want a big tool.
Going in the bush to camp for long periods? The big tool is a real asset, along with the knife.