I have +1000 hrs in RV-4 I built in late 80's and flew through the 90's, sold. I understand (timing interesting as you in WI and it's getting cold) you want to keep hot air from going into the cabin and making air vents from getting hot (which I don't quite understand**). Assume first RV you just bought recently. I will tell you what I did.
** Usually hot air just comes out behind firewall in cabin and lofts up. Not sure why your Stein vents are getting hot. Sounds like hot air is routed direct to those. So my advice is standard RV-4 set up (as best as I remember, sold the plane over 20 yrs ago). Too much heat was not an issue in Winter. I did a coast to coast flight winter and was able to stay warm with jacket thrown over my shoulder and over the stick to allow hot air from under instrument panel to keep me warm during long XC at altitude in fridge weather. I was solo. The heat was adequate (again freezing temps on ground, flying at +8000') was not enough to warm the whole cabin. I had warm clothes' on, warm stocking cap...
FIX: I have one sure fire way to "fix this". Seasonally remove the heat ducts (assume 2" SCAT, orange flex hose) from baffle to heat muff and from heat muff to firewall heat valve. Block everything off.
You do want some air flow in heat muff even when not using. When you have a standard cabin heat air goes through that heat muff and is vented into the engine bay and out bottom of cowl via the firewall heat valve.
Remove SCAT from exhaust heat muff but best remove exhaust pipe heat muff completely. It is worth while to do so every year to LOOK for damage to muff and pipe. CO2 poising anyone. This should be part of your CONDITION INSPECTION.
Block off air outlet back of baffle #3 cylinder. You may need to use metallic tape and/or make a blocking plate.
Block off Heat valve in firewall,. How? Taking valve off is too much work and likely not practical, typically riveted on. You have that 2" opening where SCAT hose was removed. You might be able to buy off the shelf stainless steel cap. I googled it, seems like they are out their, but fit don't know. You could cut a round metal plant (aluminum, SS, steel) and use SS tape and attach it.. The heat valve "seals" and you might think you don't need it. Again fumes and fire, I would be overly cautious. I would consider a stainless steel cover with tabs folded over around circumference, and attach with. hose clamp, and high temp RTV to seal it for fire and fume safety. It can be removed when you restore your cabin heat system.
SAFETY. By all means keep fire and fume integrity. Yes you will need to make some parts. Once you have them you just put them in, take them out as needed. . CO2 detector is a cheap add-on and big safety bonus.
DONE. Save weight, no heat coming into cockpit. Winter time (coming like freight train in WI) you will need heat. In spring reverse the steps. You can do this job with practice in 2 hours from time taking cowl off to cowl back on (not including making parts first time). I removed mine in summer. I combined these spring and fall events with inspections and oil changes.
PS BTW you know you need to "annual" the plane yearly for safety and legality, called a "condition inspection". As you also know you did not build it, so you will need an A&P (no IA needed) to sign off, but you can do a lion share (or all) of the work with A&P supervision. I suggest you get an RV expert A&P and/or builder/owner who has maintained RV's for a long time to help with hands on with your first "Condition Inspection". There are two book on Van's aircraft RV maintenance and pre-purchase of Van's RV's. You have to know what to look for. Also maintenance is OK for you to do even non consequential modifications called "Minor". By definition "minor" is any change that does NOT effect any major system or airplanes performance and control characteristics. I personally think cabin heat "INOP" and disabling the system is MINOR. BE SURE to put an INOP sticker on your cabin heat control. Start to mess with Prop, Engine, Fuel modifications those are major. The FAA wants to know and you may have to go back into PHASE 1, which is a restricted period, Phase 1 you can look up, and will be FAA defined, typically no passengers, limited geographical area for specified flight time to verify changes (flight test persay). Once you fly off that restriction (if FAA deems needed) you just log it and put plane back into Phase 2. No need to contact FAA again. Seriously you are in WI home of EAA and sure there are smarter people than me to help you, but HELP you should seek out in regards to another few sets of eyes and better ideas. Do you have a POH or AFM with all the limitations? Are the log books right? I have seen too many kit planes 2nd and 3rd hand that don't have even basic ARROW paper work (Airworthiness, Registration, Radio (needed for Int'l Canada, Mexico), Operating Limits and W&B. I would put this bad boy RV-4 on scales. I see RV's with W&B BOW and CG way off. NOTE RV-4 is a delight to fly solo. With 2 up also a delight BUT the stick control in pitch gets very light light light. Landing may not require little to no back pressure to flare and a tad more speed. In my RV-4 case, with girlfriend, baggage, landing I would hold a slight bit forward pressure as I had run out of nose up trim. This was at aft CG limit. Solo no bags different. Respect all the limits and no impromptu aerobatics without extreem caution, reading (a lot of info out there) and preferably get training. To legally do two up aerobatics requires two parachutes (and in class G airspace) and altitude (3000' recommended). The aerobatic weight of the RV-4 is limited. Most RV's are over Van's BOW target, so it is a solo aerobatic plane to stay within aerobatic limits. Remember Van designed this 40 yrs ago for a wood prop, O-320, and a very basic day VFR panel, may be some lights for night. Fat RV's are common....So weigh the plane when you get a chance.
