Chris is worrying about being stupid… 

stupid would be not asking the question though. 


Some interesting answers
However, my first thought was ‘why would I do that?  a four inch blade would be
amazingly hard to use.  It’s a panel raiser - so the idea is to leave a
‘perfect’ finish on the panel - evidently for the highest class of work.  So,
like our highly tuned final smoothers (we all Do have a 4 or 4 1/2 or YBI tuned
to a half a gnats a*ss don’t we?) it would only have to make a couple of passes
to follow the smoothers.  Maybe that wide blade is usable after all.
Back to the question then.  
We’ve modified is slightly, in that we now want it to take a full width,
gossamer shaving.  The requirement for this is that the edge is a straight line,
no camber, and the the two edges are polished, to make the meeting edge as fine
as a molecule (ha!)  Having a flat back isn’t so much a need as having a
polished back.  If we were to polish the back, in order to get that sharp edge
to maybe a single atom, whilst it isn’t perfectly flat, we can still achieve the
miracle edge of gnatarsness.
You undoubtedly need a jig, and scary sharp is probably the best bet, because
you can easily change grit, and continue down to, say, 2500 before moving on to
a lapping plate and sub micron diamond dust.

For myself, I’ve already declared myself in the ‘sharp as needed’ corner, so I’d
just just run over the scary sharp or equivalent regime, and give the back a
good rub down on something each time I resharpened.  You get there in the end.

Alternatively of course, a jig and a surface grinder, maybe even just the side
of a wheel, taken carefully, if it really matters.
Or, of course, follow the regime of the previous owner, and add in a bit of back
flattening each time you sharpen.  It’s really all down to your personal
preference.


Richard Wilson
Yorkshireman Galoot, in a fine sunny northumberland


and for Jeff.
4 - standard size smoother
4 1/2 - a bit wider standard smoother
YBI - Yuppie bastard infill - all manner of ridiculously expensive hand made /
cast / exotic versions of a standard smoother which can produce the same finish
as a well tuned 4
gnats ass - a very very very tiny linear measurement, slightly more than the
thickness of a shaving from a well tuned 4