My initial list
When I'm looking at a -4 to possibly purchase, I like to get answers to these questions up front. Some (not all) are deal-breaker questions:
1) Plans built or kit?
2) What is the Empty Weight?
3) Long gear legs or short?
5) Is there a:
Crash History?
Damage History (e.g. firewall bending hard landings)?
Fire History?
6) What thickness aluminum used on the elevator skins?
7) Footwells in back?
8) Have all relevant Service Bulletins been applied?
9) What is the full engine specification (if not specified in the ad)?
10) Any rpm limitations in addition to the stock engine specs?
11) Day only or Day/Night VFR?
12) Year built or kit year vs certification year.
13) Full prop specification
Just the other day I was looking at a C/S propped -4. The ad says it has a Hartzell HC-C2YK-1B – 1 hung on an O-360 A1A. Nice.
But I did a search on that prop to get it's diameter and found this:
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Hartzell HC-C2YK-1BF/F7666A-2 Propeller on 180 Hp Lycoming engines
equipped with Electronic Ignition or FADEC
The Hartzell Propeller Model HC-C2YK-1BF/F7666A-2 has been vibrationally approved per FAR23.907 on the standard production Lycoming Engine Model O-360-A1A, and similar models,rated at 180 HP at 2700 RPM with a restriction to avoid continuous operation between 2000 and 2250 RPM.
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This came from:
https://www.vansaircraft.com/pdf/Hartzell_c2yk.pdf
There's another PDF on this here:
https://www.vansaircraft.com/pdf/Hartzell_c2yr.pdf
I'd have to investigate this further but would be a deep concern that I can't run continuously at 2100 rpm.
My list grows as I learn more about what to look for. And I have to say that the more I look, the more an O-320, 160hp, wood prop sounds best. Unless it has long landing gear legs in which case I'd consider a bigger prop.
There is an article in a recent Kitplanes that talks about 35 years of the RV-4 and there's a section that talks about what to look for when buying a used plane.
Hope this helps.