Brian,
  Since I was the one to mention awhile back the high wing loading you'd
have by upping your GW to 4100 (or was it 4200) pounds, let me elaborate on
my concerns, which aren't restricted only to wing loading.
  As long as you have ample speed, high wing loading isn't so much of a
problem.   But when I think of a IV or IV-P, I think first of a plane that
needs a lot of speed to fly relative to most of the GA fleet.  By
arbitrarily raising your GW by 1,000 pounds, you make your plane into what
in the military is better known as a lead sled.  It goes fast, but absent
that speed or more importantly, absent power, it'll come down like a clean
safe.  (Unlike the IV's though, most lead sleds are equipped with ejection
seats.)  Get anywhere near the backside of the power curve in a heavily
loaded IV-P and it'll decel quickly and possibly paint you into a corner you
don't want to be in.  Glide ratios are scary.  With the Walter, you have
lots of power to get out of that corner, but if you get slow, you have
torque to deal with in trying to keep the plane upright.
  Of greater concern to me though is the question of the gear and brakes'
ability to bear such a significant increase in weight.  If you have to rely
on the stock brakes to abort a 4200 # takeoff for any reason, it could be a
real eye opener.  If you have to land heavy and plop the plane onto the
runway, you could damage your gear, bottom out your nose strut too hard,
overstress the gear box or the points where it attaches to the airframe, or
open the possibility of a whole host of other structural damage issues.
  I questioned whether you had run the increased  weight issue by anyone at
Lancair and am curious if you ever did.  Most of us have taken off a little
heavy at one time or another, but not 900 or 1000 pounds over the max gross
weight in the POH.  Just because you have a big engine doesn't mean the rest
of the airframe is capable of handling such a big increase in loads.  I just
hope that before heading off at such a weight you'll thoroughly investigate
the aerodynamic and structural consequences as well as the risks of doing
so.
  Skip Slater