I'm with RocketBob in that the numbers do not mean a lot. You really REALLY need to sit in both to know what will work for you. More importantly, you need to determine WHAT you want to do with it in the long term. Performance is similar...but one, the 4, is more focused, the other, the 8, is more general purpose.
Subjectivity and perspective has a lot to do with the choice as well. Even more than body size. I am 5' 10" and 210 pounds. I can fit in them all "comfortably" so to speak, although you could not pay me to fly a 6 for long. Sorry, but its a tight squeeze on the shoulders with two adults - And thats part of the subjectivity. I've live with a tight cockpit for years in the form of a Luscombe...If you are going to fork over what one of these things costs, weigh all of the factors.
I've wanted a 4 my entire adult life. But when I sat in an 8, my heart sank (CHA$$$$ing) because there was no way I was going to get a 4 after that experience. I fit in the 4 fine...BUT (and here is the subjective feel) - I've been flying Luscombe's for 25 years. The proverbial phone booth with wings. Tight cockpit with no room if you have large passenger next to you - and the back of the 4 is like a sardine can for anyone over a certain weight. I did not want to trade a slow tiny cockpit for a fast tiny cockpit. The 4's front isn't all that large either and I hate having the canopy in my face, literally. Nose inches from the glass. No idea why, but it just bugs me silly. Never thought it would! Whereas after the first two hours in the 8, I do not even notice the roll bar. It bothered me at first, but you have room to MOVE. So you end up ignoring it. Better still, you can put things up there on the glare shield temporarily and that comes in quite handy! SPACE. SPACE is good. The 4 left me feeling boxed in. Visually Stunning mind you, but boxed.
So I bit the financial bullet and got the 8 instead. But only after several considerations that again, are based in perspective and subjective view and on what you plan on doing with the thing:
Performance. Its a wash. Both the 4 and the 8 are amazing aircraft and both perform well in all aspects a sport pilot can ask for. Speed, climb, cruise, basic acrobatics, alls good.
Passenger Comfort. If you plan on flying the spousal unit around, she better have room to get comfy or you will be misersable on every X-country. That or she just won't go...hmm....ok...need to think that one some more! HA. Anyway, the 8 is so large I can literally rest my arms on the sill, relax and sleep back there. So she has no room, lol, to complain. I spent 6 hours in the back of an 8 a few months ago and was never, not once, uncomfortable. Thats saying a lot.
Storage space. Neither have accessable in-flight baggage areas, so room for charts and all the other garbage we take along is a plus. Both have places too stuff gear, but the 8 seemed a bit better. Leg and shoulder room seems massive by comparison. Even the glare shield on the 8 is useful. Panel realestate is a little better on the 8 as well. I agree with the comment about "feet together" in the 8, but its not bad. I totally agree the stick is too friggin low on the 8. On me it works however, because I just rest my hand on my leg and only need to move my wrist. Didn't like it at first, but again, it was totally subjective. Its far different from the Luscombe, where your entire arm is free and flapping. Now I am really starting to like it.
Flight envelope. If you plan on flying a fat friend in the back, get the 8. We've had 230 pounders in the 8s with no issues at all. I've heard more than one -4 pilot complain about CG with 160 pound (or heavier) guys in the back. That was one of the biggest things I didn't like about the 4. All my buddies are 6 foot and 180 to 220. Easy fit in the 8, but some friends with 4s complain about aft CGs and a "balancing on the head of a pin" feel in the 4 with a real fatty in the back.
Conversely, if you plan on flying alone most of the time, the 4 still makes a ton of sense. Cost a heck of a lot less for starters. And its not THAT small inside.