Twenty or so years ago, after I purchased a LN4, I converted a junker Made in
England Stanley #4 into a scrub. This was long before Paul Sellers and YouTube.
I filed the mouth open to 3/8”, ground the blade to 3/16” from center to edge (I
think the radius is 6-8 degrees), knocked off the corners of the chipbreaker to
gain full extension and slightly filed the top of the chipbreaker to prevent
jams or clogging. Full disclosure (blasphemous as it may be): now that I’m 72,
dimensioning rough lumber by hand is no longer a labor of love, so it doesn’t
see much use anymore. In fact I noticed the blade could stand some touch-up when
I was taking the photos. At any rate, it works fine. Maybe not quite as good as
a built-for-the-purpose scrub, but all it cost was about an hour of my time.