Right, a #4 is about 9" long, a #5 is about 14" long. The #4 is called a "Smoother" and the #5 is called a "Jack Plane"
As for which is better.... It depends... especially on which one currently has the sharpest blade.
I have found that I really like the #5 and larger planes for Guitar Plate flavored work -- Tops, Backs, and Sides. I really like my Miller Falls #22 (Same as a Stanley #7) for thinning down plates. Its mass makes it easy to bull through material. The #4 has a place too-- it is much smaller and more maneuverable and less of a workout. A home made toothed iron is definately a must for Guitar plate work.
A good Low Angle block plane is also handy for neck work, neck blocks, tail blocks, and leveling the sides.
My only brief caveat with used planes: Do you want to work wood or Fettle old planes? This is the danger.... as you can get easily end up spending countless hours on "Restoring" and admiring old planes rather than actually using them..... I know from whence I speak.... I have 6 Pre-1950 planes, and almost got hooked on Fettling and Rust collecting... but Guitar building actually brought me back over the edge....
If you are possibly pondering a new plane -- Check out Wood Craft's Wood River line of planes. Probably the best plane value for the money right now. Mine were truly ready to go out of the box with 1 quick sharpening.
Good luck
John
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