Due to the nature of this reactive deck, most of the time you're going to put it into Taunt mode. In some other versions of Druid, notably Combo style, you might see more Charge to add more damage. For this deck though, you're usually looking to set up Taunt Walls so that your late game can win it for you.

That said, there are a few situations I've used Charge. It comes down to whether or not you can gain some value out of the 4/4 doing 4 damage to something. Otherwise, a 4/6 might just "for free" waiting for the opponent. Some examples:

• Against Mech Mage when Mirror Entity is in play. Giving Mech Mage a 4/6 with Taunt not only gives them a bigger minion that has initiative (it attacks first), but it protects their smaller units. And Druid is really bad at taking out 6-health minions. You'd rather create a weaker 4/4 Charge where you have initiative, and you can use that 4/4 to either kill a smaller minion (ideally 2-attack or lower so you don't get pinged), or trade with the 4/4 you just spawned.
• Against a deck where you expect them to remove a 4/6 Taunt before you get to attack with it. For example, when you face Mech and you haven't seen any Fireballs yet. You want to squeeze value out of the DoC, so putting up a 4/6 Taunt that gets fireballed is less preferable to a 4/4 Charge that clears a 2/3 minion. This can also apply to Shaman with Hexes.
• Against any deck where you need to remove a threat. Sometimes you just need to kill a Knife Juggler, Leokk, Azure Drake, Gadget'zan, Antonidas, Flametongue Totem, Mana Tide Totem, Acolyte of Pain, etc.
• Against Freeze Mage. It's trickier, because 4 health dies to Flamestrike, but the Charge gets you 4 extra damage. This one depends on the state of the board and the Mage's health. If the rest of your minions are 4-health or lower (or maybe if 1 has 5 health and it's turn 9) you should consider putting up a 4/6 instead to survive Flamestrike.
• Proc'ing a Trap. Especially if there's a Freezing Trap, and you need to keep a bigger Taunt in play, you might want to bring back the 4/4 Charge at 7-mana than the Ancient of War you just played.
• Stripping off Armor of Warrior. Does the Warrior have 4+ armor already? Well, Shield Slam (possibly with Armor Up) kills your 4/6 DoC. Why not get some value out of the 4/4 and maybe even put it out of Shield Slam range?
• Of course, if it gives you Lethal or sets you up for Lethal on the next turn.

Just some examples. I'd say 80+% of the time, you'll see a 4/6 Taunt from my DoC. But there are certainly times when the 4/4 is right.