Well, Zen, you see, as I said, all of this depends on what sort of
work you are doing. Keeping that in mind, though, and the issue
being that you can only get ONE more file, no more, then: You have a
3 file, which could even be towards a #4 cut as it is marked. I
use a #2 as my finest file to begin with, this being for work in gold
In skilled hands a #2, and certainly a 3 or 4 will be quite flat (in
a flat space----) and fairly smooth - about comparable to 100 grit
sandpaper or so. But let us suppose that you are finishing a raw
casting, or filing edges flush after soldering, which are two common
tasks one would use a file for. Are you going to set out removing the
sprue remnant with a #3 file?
It will take you 1/2 hour, and put excessive wear and tear on that
poor little file. A #3 or #4 is intended to true up edges that are
already "in place", not for roughing. In short, you could call them
finishing files. So, my thought (not so far as actual "advise") is
you go rougher with the files, even to #0, and, since your #3 is
comparable to 100 grit, give or take, then use sandpaper or Cratex
or the like from there for the finish.
Plus, there is not a lot of difference between #3 and #4, and #6 is
so fine that you can barely feel the teeth on it - very few people
actually need a #6 - superfine modelmaking and high precision work,
like aerospace stuff. Some might like to use them, and have plenty
of files, but to buy only one more?
In a nutshell - getting a #4 that's not so different from what you
have is redundant, and only marginally useful. Buying a #1 or #0 will
speed your work greatly and give you other options - carving away
material that you might not attempt with a fine file, for instance.
Or - spring for two files and get both..... Finally, don't listen to
anything I say.
LOL..... Go into the file store, pick them up, feel the teeth and
ask yourself, "Is this useful to me in the work that I am doing?"