Also, whether or not you like Cobbler (or Nocturne) has nothing to say about card balance. Costing a card with comparable or higher strength at 4$ would be imbalanced even if Cobbler didn't exist.
That said, I think you should not exclude Reserves from Sled Dogs, and instead should exclude Victory cards (we don't want to gain Islands or Castles straight to the Tavern Mat). In fact, I think you could just make it "non-Victory". I also think that your grammar and wording is off. In Dominion, a card you add to your own is always "gained" (barring Masquerade), which by convention makes it unneccessary to mention "from the supply". So, I would word the card:
Sled Dogs, Action, 5§
+1 Card
+1 Action
Gain a non-Victory card costing up to 4$ onto your Tavern Mat. Turn your Journey token over (it starts face up). If it's face up, put a card from your Tavern Mat into your hand.
If you want it to be a 3$ cost, I suggest following Holunder9's wording (small alterations by me):
Sled Dogs, Action, 3$
+1 Card
+1 Action
Turn your Journey token over (it starts face up). If it's face up, gain a non-Victory card costing up to 4$ to your Tavern Mat. Otherwise, discard a card from your Tavern Mat.