I would suspect that 4 bears + tree would be viable depending on the mobs, but it would take quite a while versus 1 bear + 4 cats. Basically, if they all die before the tree runs out of mana, then you're golden. You'd have to play around with it a bit to find the sweet spot, though, as that's hard to estimate and depends on gear, enchants, etc.

Generally, though, the biggest problem with leveling druids versus leveling paladins is the lack of really good passive healing. With kitties, the passive heal from ILotP helps but it isn't anywhere near as good as Divine Storm + seal of light. Really, though, since instances are gravy and not really required for leveling, 5 feral should work fine. If you have real problems with a particular instance, just level once or twice and you can go back and faceroll it.