Minor quibble, but 2E and earlier forms of D&D many times didn't use PC classes to create NPCs or monsters, either -- they sometimes did, but if it wasn't a human or "demi-human" race (like elf, halfling, or dwarf) they were a stat block not too dissimilar to the way 4E does it. 3E is the first version to always treat monsters with the same gradient consistency as PCs.
I'll agree with your major point, though -- if we were, for the sake of argument, going to translate Merlin to 4E PC stats, the important thing is to figure out what you're trying to emulate, and then pick the classes or stats that best model that. In Merlin's case, I'd say look at the abilities you're emulating -- are we looking at the old stories, from Both La Morte d'Artur and even prior (some of the Ambrosius and Merlinus Caledonensis stuff that preceded being linked with "King Arthur")? Or are we looking at the lgends closer to graphic novels and comics, or the D&D stuff?
If we're looking at the written legends, then Merlin had a lot of confusing powers - I'd fathom him an illusionist almost before I'd call him a Wizard. Actually, the Merlin as mentioned in folklore (depending on sources):
- Had giants for friends, who helped him build Stonehenge
- Recited poetry
- Wrote prophecy (or maybe had prophetic visions, I'm a little unclear on this)
- Could shapechange into multiple forms (animal and man)
Not so much on the Meteor Swarms, Time Stops, and Cloudkills, though.
Based on the original stories, Merlin in 4E terms would not be a wizard at all! He would probably instead be... get this... a Bard, near as I see it, both in 3E terms (the 3.5 bard could do a lot of this) and in 4E, either a Hybrid Druid/Bard, OR a Bard, but with Druid Multiclass feats.
Neat thing is, if you really wanted some fun, have a look at one of the Bard Class features:
Multiclass Versatility You can take MORE than one multiclass! So, Merlin the Bard, takes Arcane Initiate, Initiate of the Old Faith, Then the Full set of
Novice Power, Acolyte Power, and
Adept Power, and Old Merlin can use damn well whatever funky spell, evocation, or utility you please for him for story purposes! His Bardic abilities serve him well for things like making buddies with giants, having minor prophetic moments to help allies in combat, and even covering him and allies with mind-befuddling illusory guises.
Now, Merlin was supposedly a Cambion, a half-man half-demon. Sound like Tiefling, anyone? Hmm, Tieflings get a bonus to Intelligence and Charisma... Perfect for the legendary Bard/Wizard! He'd just need to nix those damned Hellboy-Sized horns on the stereotypical Tiefling head, maybe trim that tail down (maybe the local barber could dock his tail...

) Or, leave him with the D&D-style horns and tail, to come as a shock to PCs when they hear these legends about the "famous" Merlin and get in for rude shock when they meet him in person...
Griznuq, I think there may be some confusion here because, based on your link, you are trying to create an NPC, not a PC. Specifically, you are stat-ing famous Arthurian character Merlin as an NPC.
4e has a big change here from earlier editions -- monsters use different rules than PCs. Monsters don't have player character levels. So, while a multi-class Wizard/Druid is an appropriate way to represent Merlin in earlier editions, in 4e he would simply be statted as a monster (likely a solo or elite). Drizzt, for example, was statted out in a Dragon article (or Dungeon) as a level 21 solo, not as a "Ranger." You did this accurately in your blog post, giving Merlin monster stats.
However, when you ask how to convert "a 3.5 wizard/druid to 4e," to a 4e player, it sound like you are talking about PCs. So you got a lot of (good, thoughtful) responses on how to turn a wizard/druid PC from 3.5 into a 4e PC. But it seems from your link that what you really wanted is NPC stats for Merlin.
Anyway, I'm no master of monster stats, but your version looks reasonable-ish (and appears to be made using the monster builder, which is good). However, someone like Merlin is so iconic that they should probably be a solo or elite, and therefore more powerful. Solos and elites have various other complicating factor in their design (mostly having to do with economy of actions compared to standard monsters) which you can get more advice on from me other posters, if needed.
Note: if you don't expect the party to "fight" Merlin, you don't need any stats for him at all. Non-combatant NPCs in 4e generally don't have statsl, because they simply do whatever the DM wants to have them do to advance the plot -- casting rituals, using "skills", etc. as the DM desires, not according to set rules. If the party needs to interact with Merlin in a challenging, non-combat fashion, you use skill challenges, in which only the players, not Merlin, make skill rolls, and the difficulty levels are based on the PC's levels, not the NPCs stats (since the NPC generally doesn't have stats outside of combat).