I'll preface this by saying I'm late to the party, didn't bother reading anything except for the first page, and my opinion probably means jack shit because I'm not even an active player. I'm just an old vet ranting.
Nathan and Cutt3r basically summed it up. RS4 was simple, light RP, and most of all it was fun. It wasn't like other RP servers -- one didn't have to write a 5 page character background to be whitelisted to play, one didn't need to have a second monitor with script commands on the side, one didn't need to drive around for 40 minutes just to find someone to speak to. As far as servers went, Argonath was "plug & play, take things at your own pace". And if one wanted to stay and become an actual member of the community, all one had to do was play for a week and know that the President was named Gandalf. Maybe it's the nostalgia talking, but RS4 Argonath was a simpler time. It fulfilled a unique niche in the SA:MP RP community.
Sure, some of the RS5 scripts are dope. The notoriety system was my favourite addition by far, at least for the brief period I was active again. It actually managed to flesh out a difference between criminals and civilians, instead of just the "white name / orange name" system that RS4 had. I also enjoyed the meth cooking, the money laundering, the new property, group and vehicle scripts. But the list of what RS5 ruined far surpasses the list of what it added.
Remember when one could just drive around, relax, and not be forced into anything?
Remember when just the idea of tazers and IC/OOC chat would you get ridiculed at best, and witch-hunted at worst?
Hell, remember when one could safely have a criminal roleplay session in-game without the FBI using footage from their 360° 8K 60FPS HDR microscopic pen cameras to sue one for half their shit on the fucking forums?
RS5 didn't just change scripts, it changed the entire feel of the server. It butchered the playerbase and it killed the server both figuratively and literally. It allowed a vocal minority to become the majority, which led to taking the entire thing into the direction we're at now. You can say "but SA:MP was dying" all you want, it won't make any of this less true. One moment we're at 200+ players, the other moment we can barely break 40. Ever since RS5, the server has been a poor caricature of its former self. An empty husk. No players, no Argonath vision, no soul.
With all this said, the glory days of RS4 will always be in the past, nothing can revive them. The community itself has changed and people have moved on. RS5 launch was legitimately the worst thing that ever happened to the server, and no matter how much it was "fixed" during the following years, one can't simply glue together something that broke into a million pieces. Part of me believes that RS6 is the natural and logical step forward, but another part of me believes it's just a re-package of the mistake that was RS5.
I'd vote for RS4 just because I'd actually be willing to check it out again and re-live what memories I have. I simply have no faith RS6, because I actually had faith in RS5 and got severely burned on its release.