Not a safe plan.
Ultimate load is where structual damage occurred. So loading for +3.8 and -1.5 SUSTAINED requires essentially +6+and -3 Ultimate to allow for the FAR safety requirement. (not necessary in our application.. but a great guideline)
Also, keep in mind that the Model IV is the final evolution of the Model 1..
So the Gross weights have increased from:
Model 1 - 850
Model 2 - 950
Model 3 - 1050
Model IV - 1050
Model IV - 1200
And before anyone asks.. no... one cannot increase the gross weight of their 950 model 2 to 1050 or the 1050 model IV to 1200. There was never an "Approved" modification to allow the changes. Yes there was a written document by Denny Aerocraft that allowed a 1050 Model IV to 1200 when operating on floats only... certain changes needed to be made.. and when back on the gear it was a 1050 gross aircraft. To the best of our knowledge that modification was never structrually tested.
Before the Series 5 came into existance.. The Model IV was evaluated for another change not only in gross weight but also to allow for larger (heavier) power plants like the Continentals... It was determined that the IV had reached a evolutionary limit and would require to many changes to accomplish the desired changes and a clean sheet design was started for the Series 5.
There are other major factors in gross weight calculations.....
So while some have "gotten away" with flying over the recommended limits it is not a safe practice. and that holds true for Cessna's Luscombs, Pipers etc...
That being said.. The builder is the manufacture and most likely the test pilot.