I want to put up a new fluorescent fixture in my basement. Right where I
would put a box for an outlet, there is a cable for an appropriate circuit.

The circuit was put in pretty sloppily, so I was able to get 4" of slack
right where the new box would go. Is that likely to be enough wire to add
an outlet? Seems adequate, but I don't want to cut it up and find I needed
5".

If it turns out not to be enough, I would have to either put in a junction
box to add some wire, or pull out the last 10' of cable and replace it with
an 11' cable. That would be a lot of work since it all passes through floor
joists, but it would save a junction box. Is it worth it?