Disclaimer: I have a very limited knowledge of Earthdawn so I might be completely off-base in my premise.)

Ok, I seem to recall someone making an off-hand comment that since Metahumans were Name-Givers one of the Fourth Age holdovers could in theory Rename Seattle and change its very nature/climate. (Of course I would assume that doing so would make the Fourth Age Holdover's head explode, but you get my point.)

So I guess my question is how exactly did it work in the Fourth Age and more importantly what are the impications and proposed rules for the Sixth Age and Fourth Edition?

Well I can give you an example of renaming for a person/Name Giver if not a locale. For a Name giver, they generally go through a Renaming Ceremony, whether it be an elf coming of age, or an adept attempting to basically reset their pattern. They formally takeon their new name, go by it, that's their name form then on. There's probaly some more religious or mental mumbo jumbo that goes with it, but that's the basics of it.

Once they are effectively renamed, all threads that they'vewoven to people and objects are broken, and adept powers they've gained, including ranks in their talents are lost.

So I suppose as an example, an adept in the 6th world that believes in Naming and Renaming that went through a renaming ceramony would start back at their beginnning magic atribute and lose any initiate grades. They'd have to beleive something like that would work though.

I can think of one example of a place that was renamed. Wyrm Wood became Blood Wood, which required a massive magical ritual. Obviously, the more set a place is in a group of people's mind, the harder it's goign to be to Rename as well.

Well, in Earthdawn it helps to remember that a Name (what we would call a True Name) doesn't describe a person, place, or thing - it defines it. Their Name gives rise to their Pattern. All that a person has ever done, or has had happened to them, is contained in their Pattern. When you know a Name, you can weave a thread to the Pattern that Name gives rise to, giving you a connection to that Pattern and some advantage with regards to it.

Naming an item or place can be easy, but it doesn't stick if nothing in particular defines that item or place - calling a sword Dragonslayer doesn't make it capable of killing dragons, but if you use it to slay a drake, a wyvern, and a dragon-headed chimera, it will start developing in that directing.

ReNaming, by contrast, is much more difficult. Most of the time you'd have to collect all of the Pattern Items - objects that are integral to the history of the Pattern, and which you can use to weave threads to the larger Pattern - though sometimes ReNaming can be accomplished by a particularly powerful application of ritual blood magic (with the significant possibility of things going terribly, terribly wrong).

The consequence of a successful ReNaming is to completely erase all ties and traces of the older Pattern - threads to other Patterns would unravel, magical abilities would be lost, curses broken - it would be like the person, place, or thing had been born anew. Some Namegivers who are ReNamed may not even remember their old lives, and none retain their magical abilities.

In Shadowrun, the closest we have to real Name magic is the relatively limited use of True Names to call and bind Free Spirits.

That doesn't bother us holdover types though. We just figure that the mana level isn't high enough for stuff to just become magical because something cool happened to it.

Hmm, so in theory even at current Mana Levels someone with extended knowledge of Naming Magic might be able to used a metahuman's True Name as a Ritual Link, other then that it hasn't really come into its own yet then? (I understand that it isn't Canon by any means, but its more of a what-if question.)

Ok, would someone's True Name be as simple as their given name coupled with a good look at their Pattern/Aura or is there more to it then that?

Also since Renaming would basically rewrite someone's Pattern I would assume that it would be an effective if very costly way to rid oneself of a Horror Mark as well.

Also in Shadowrun Terms a Horror Mark would basically just act as a perfect Ritual Link right? Or should it be even more powerful and give the Horror Magical LOS to the character at all times?

You've already hit on the truth of the matter. Many individuals within Shadowrun are capable of performing a renaming ceremony for themselves. When a magician initaties, it's possible for them to 'reset' their magical pattern. When that pattern is reset, prior ritual links of any kind fail to work, and I don't think it would be a stretch to suggest that foci which were not near the magician during this would become unlinked as well. I can't say for certain that a horror mark would be unlinked by that act, but I ruled that it did, in one of my games.

In terms of true name magical links, it's even simpler. You have to have the Linking metamagic (originally in SoTA63, I believe), but then any item representative of the target can be used as a link.