Basically a lot of existing D&D campaigns won't transfer to 4e because of the changed assumptions. My old 1e/3e campaign moved to 3e OK, but I'd never have been able to move it to 4e - all the classes are so different.
The trick is to divorce the concept from the terminology for the mechanics of another edition. An old character, as a concept, isn't defined by his class title, but by his abilities. If you look to see what your character could do before, there is a very good chance that you can convert him to 4e with the options available, some reskinning (which is encouraged by the system), and maybe a bit of DM love here and there, but this last bit isn't often required in my experience.
By way of an example. My group is coverting our old 3e high level campaign (in which I play while another DMs) to 4e. My character in that game proved a bit problematic when he seemed like he would be the easy one of the group to convert. It's an evil campaign and the characters are -
My 17th level CE warmage
17th lvl Cleric of Nerull
17th lvl half-demon warblade
7th lvl rogue 10th lvl assassin
The cleric favored big battlefield control and necromancy, but with PHB2, we have this covered with the Invoker and the necromancy powers from Dragon. We all agree that the low-mid level undead army in tow is a good thing to move past, so that's not a sticking point at all.
The warblade will either go straight fighter or more likely swordmage, as he played up the magical side of the warblade before. He was somewhat tempted by warlord, as he is a general in the army of the evil emporer we serve, but that was him getting hung up on the name. By his abilities, either of the above defenders fit.
The assassin would be easy, but that player has moved on and a new player is considering an evil bard, which should be interesting. We will either eat him alive or he will quickly learn that my slightly unhinged caster enjoys songs composed of his foulest deeds and will find a good niche in the group.
As I said, I assumed with warlock and wizard in the game, mine would be an easy convert, but when I thought about what made him tick as a character, it proved a bit more tricky. He was CHA based and this wasn't just a stat, his ridiculously high CHA was central to the character. His social skill was legendary. He loved to go nova, utilizing the warmages big AoEs and liberal application of metamagic through feats and rods. He had peered beyond the stars and was rather unhinged for it and prone to wild bursts of barely controlled magic (he used his advanced learning to snag some more prismatic spells than the paltry pair on the warmage list, and constantly used a rod of wonder). He worshipped Erythnul.
For AoE the obvious choice was wizard, but the high Int undermined the concept. So warlock became obvious, but hardly any AoE. Star Pact would take care of the Old Ones vibe. And he loved big damage so striker was perfect, curses would fit even if he didn't do them before. But the only paths to AoE were wizard and cleric. Cleric seemed the better choice to cherry pick a couple of the decent AoE cleric powers, and wisdom was easier to justify in the character than intelligence (he had a decent wisdom anyway).
Then PHB2 came out and the sorcerer ended all that, chaos sorcerer - cha based, striker, decent AoE selection, has the wild magic covered obviously, Wild Mage paragon path, might multiclass warlock for a couple of Star Pact powers just for the flavor, not sure yet.
Point is, the sorcerer is perfect for the concept, even though it is called 'sorcerer' and not 'warmage'. Point is also that I had it worked out serviceably, before the PHB2, simply by divorcing the character concept from the 3e game mechanics and terminology.
Silverblade, what you are looking for is not a druid. You are looking for a nature based caster who can heal. In 4e, that is the shaman, not the druid. The shaman fits well with elements of Dark Sun as others mentioned, and it does what you want it to do, don't defeat yourself by getting stuck on semantics. He's a primal caster, with ample healing, nature based spells, and a druid pet.
