Problems & Ideas:
The premise seems a bit silly, and too far-reaching, to me... If the GC (Great Catastrophe) killed off all Wizards (and, presumably, all Druids, Sorcerers, Cleric, Paladins/Rangers, etc.), then why not all Dragons, Nymphs, and any other race with more magic than a 4th level PC? That's a bit much, ifya ask me.
Anyway, high level Barbarians, Fighters, Rogues, and Multiclassers would rule, for a time. Fighting and war would have become common... unless you go with your "moved forward in time" idea (in which case, you'll have to explain the return of the magical races).
Here's an idea that might tie it all together...
After the GC, Nature, herself, stepped in and took a hand in setting things right. She took all of the intelligent races, and moved then X,000 yers into the future, where they couldn't interfere. Then, she created her new race of Plantfolk in the forest, where they quickly matured to serve her. Then, she used her "Treewards" to help the ailing planet recover, seeking out survivors of the magical races (usually too young at the time of the GC to be able to take care of themselves). Some races had to be gated in from elsewhere or when...
After X,000 years, some sort of equilibrium has been re-established. The forests, etc., have reclaimed the old civilizations, and new ones have arisen. The magical races are as common as they were, but the islands still move in unpredictable patterns. Nature has returned to enough of a semblance of her former self that Humanity (in all its forms) is once again turned loose upon the world...
Again, Barbarians, Fighters, and Rogues rule the day, as their high level characters survived the GC, and are now here, in the future world. Wars will be common, and magic rare.
Magic items will be hard to come by, as much was destroyed, and few can now make anything. Little is left from X,000 yeras ago. All the potions and scrolls will be rotted or spoiled, and the Celtic Lemures would have picked up anything easily gotten to.
The Treewards in the forests will be the most powerful race (probably), but they exist to serve Nature, herself, and have little use for the various races of Humanity. So, they ignore them, as much as possible.
Magic items will be very rare, for a while, with new scrolls just starting to come out, and potions needing a while. With only fourth level casters, to start, permanent stuff will only be available in long-buried caches in hard-to-get-to places, or in the hands of old, experienced Celtic Lemures (Volcae).
Incidentally, Volcae won't be limited to fourth caster level. They may well be more magically powerful than Humanity! ;-p