*laugh* Trying to keep a secret plan to spring on the DM? I understand the feeling. However, there are lots of times where I will actually email my DM an idea beforehand so he can keep it in mind as one of my possible options. Especially if it involves a lot of variables. I like to let him think about the options so he can have an appropriate response ready ahead of time. Some of my plans get a bit convoluted and I don't want to bog down the game if I can avoid it. Though, there have been plenty of times where we have ideas that don't make it into our discussions on our webboard. They just come up when the dice start to fall.
But, back to your situation. I woudl definitely use the slaying arrow on the dragon it is most likely to affect. With dragons the size of what you are fighting, you may be dealing with as many as 5 attacks a round from each of them. That's why I suggest trying to use walls to avoid being surrounded. Sure, if the arrow works and you take out one of the huge dragons, you got more value for the arrow. But, if it doesn't work, and the difference in the save would have been enough to take out the large dragon, then you didn't get any value for the arrow. Unless you are feeling lucky, I would stick with the odds.
When dealing with multiple tough opponents, it is always better to focus on one at a time until it falls, or leaves. Even if you drop all 4 dragons down below 10 HP, they can all still attack with full capabilities. Whereas, if you focused on one at a time, you would end up with 1 dragon that still had 40-50 HP, but you would have reduced the offensive power against you by 75%.
Let's see, it isn't likely that the Druid/Ranger has Transmute mud to rock,or control winds. If you had that, it would be fun to drop the mud to rock as soon as the dragons landed in the marsh. Assuming you could get them to land.
You could use the Wall of Ice to encourage the dragons to attack from one side only. Then, drop spike growth in the grass in front of you. If a dragon lands, they will take some damage.
If you are going to fight this out, you want to do as much as you can to setup some control of the combat area. Just standing on the hill and swinging when you see them will not work. They are faster than you and can surround you. Since Black Dragons are immune to acid, they can just breath from each direction, you will be caught in some nasty overlaps and even the rogue will probably fail a save or two. The dragons won't feel a thing. Use your spells to block off certain attack vectors.
Use some minor summoning spells to bring in air elementals and try to whip up some wind. This will force the dragons to the ground. Use spike growth to make the area they walk through do damage to them. Even entangle might slow one dragon down for 1 round. Try plant growth as well. What are the weather conditions like? Any chance of pulling off a Call Lightning?
Just a few more thoughts...