The sit test
All you can do is sit in a RV in a RV-4 and find out for yourself. If you want a tandem the RV-8 will serve you better no doubt with your stature, but to answer you question about the RV-4.........................
Close the RV-4 canopy and sit in it for 2 or 3 hours! That should tell you. You may know with in a few seconds. Are you built like
Hulk Hogan?
I have lots of time in RV-4's. Height is not the issue, width is, especially at or just below the shoulders and down to the elbows. Van is a pretty tall dude, but of slim build.
Compairing a side-by-side to a tandem is a little hard. With a RV-4 the passenger is not competing for space, except for their feet that are down strattling you down low on your left and right. The side-by-side RVs are going to give you more room, especially if solo, but with a passenger you may be crowded, especially if the passenger is a larger guy as well. You said you flew the side-by-side already, so you kind of know what that is all about. It is a little like a C-152 but a bit wider, better but not much.
I am not as tall as you, but I have a barrel chest and big arms. I fit the RV-4, but it is a little snug in the shoulders and elbow area. You wear the plane, you don't sit in it.
That is kind of the fun of it actually. You feel like you are part of the plane. However you got 100 lbs on me. If you are super wide you will need to go to a RV-8.
Regarding head room again, unless all your height is in your torso, neck and head, head room is not bad, if you use the minimum cushion and your headset band is not super thick. The RV-4 will be OK, but the RV-8 no doubt is better in the head room department. If you where not hitting your head in a RV-6/7 you should be OK head room wise in the RV-4. I am not 100% sure but I recall RV-4's have free blown canopys which can affect the head room, because there are small variations in canopys. Also how the builder installs the canopy can make a little difference in head room. I would not be surprised if two different RV-4's had different head room. Probably we are talking may be 1 inch or so variation? This is a guess on my part, but looking at RV-4's for many years, some seem to have variation in canopy height.
If you are seated properly, all RV's put your head within a few inches of the canopy, but you need your seat belt and shoulder harnesses on all the time. I fly the taildraggers so I have my cushion as thick as possible so my eye sight is as high as possible. It gives better taxi and flight visability. Even though I am not as tall as you, I like to side high and near the canopy. You will need min seat bottom thickness.
I remember flying X-C in my RV-4 and approaching Reno at night, in a cruise descent, I hit some sharp vertical drafts with my belts loose. I whacked my head on the canopy. RV's are small planes, but you sit in a normal semi-upright (comfortable) position. It's not like a LongEZ, Glasair or Lancair 320/360, where you are in a lay down position.
Don't know what to say but the RV-4 may be tight for you. Find someone near you who will let you sit in their bird. Bring your own thin foam seat bottom to trade out if needed. Keep in mind there are some excellent hi-density foam and temper foam that can make a comfortable seat bottom with out being thick. You can talk to Oregon Aero or Classic Aero designs (both sponsors of this site) and ask how thin they can go on a seat bottom, compressed.