Dark Elves are a bit limited in which Lores they can take, but they get enough choices to make things interesting. First, however, we need to talk about the spell all Dark Elf wizards know.
Power of Darkness: This spell, and having all your wizards know it for free, is what makes Dark Elves so potent magically. You gain 2-4 Power Dice for a spell you are very likely to get off several times during the game. Your regular sorceresses can cast it on two dice with a low chance of failure, and your Sacrificial Dagger sorceress can do it with one, adding the extra if she rolls only a one or two. You can cast it throughout the magic phase to keep it going, well after your opponent has run out of dispel dice and spell defense. Even a poor Winds of Magic Roll can be somewhat mitigated by this. Also, with the FAQ, most of the negatives have been taken care of. You CAN attempt to cast spells you normally couldn't succeed on in order to get rid of the dice in the pool. It's a huge boon to your magic phase.
Lore of Dark Magic: This is the Dark Elf lore, and its primary benefit is going to be that all the spells are easier to cast because of the new magic rules. A level 4 wizard can cast even the hardest spell in the Lore on a 7+. However, the spells themselves really aren't good enough when compared to the other Lores you could choose. Also, with no Lore Attribute, you're missing out on the handy side benefits of the magic phase.
Chillwind is still a nice spell since it stops a unit from shooting, and the range is just right to be useful. Doombolt is a fairly basic magic missile, and Bladewind operates as another direct damage spell, with the ability to drop Champions as well. Lore of Fire is a better option, overall, if you're trying to go for damage, however. Word of Pain is handy because it can shut down a unit's shooting, and make them terrible in close combat. Soul Stealer is short ranged, and I've found it rarely to be useful enough to give too much consideration. Finally, Black Horror (our big spell) is difficult to find a role for, since its best against low Strength models... that we are generally alright with dealing with. I'm not sure there's any need to take the Dark Magic lore in the new edition... especially with the new toys you get from the other Lores!
Fire: The main purpose of this lore is causing damage. With two direct damage spells, two magic missiles, and an augment and hex spell that cause damage, it’s designed to put extra wounds on units that draw the wizard's attention. For your level 4, I actually think this is a good choice. The damage spells will take a few models off various units, allowing your Black Guard to handle them easily, and in addition, the non-damage spells are great.
Cascading Fire Cloak is probably the least useful of the spells in the lore, but will make rushing into combat against the unit a little dangerous for your opponent. Fulminating Flame Cage is a great way to kill a good amount of models, or freeze a unit in place (if they don't want to risk it). Flaming Sword of Rhuin will make your Black Guard even more dangerous in combat, or alternatively it will make your shooting more effective. Flame Storm is probably the weakest of the top level spells in the game, in my opinion, but even so, combined with the other spells in the Lore, you're going to kill a lot of models.
Metal: Lore of Metal makes a good choice, because it offers Dark Elves something they can sometimes have problems with: heavy armor. The Signature Spell does the majority of the damage in this regard, but there are a few other spells here that help out in various ways. Plague of Rust will permanently reduce an opponent's armor save, which combined with all your armor piercing is going to add up quick. The Enchanted Blades of Aiban isn't going to be that big a help to you, but an extra +1 to hit is rarely going to cause you to complain. Glittering Robe will make your units a little tougher to kill, which is a huge help with T3 and normally 5+ armor save. Transmutation of Lead will help you a little with breaking armor as well. I'm not that excite about Final Transmutation, but it can be pretty handy for occasionally tearing chunks out of a unit (if you're lucky).
I look at Lore of Metal as a good supplementary lore. I don’t think it's good to put on a level 4, but a level 2 may be well off taking this. Some of the spells are useless against some armies (like Daemons and Skaven). If you're facing one of those, you're going to have a lot of spells that just do nothing. Even so- having a wizard take it will probably help more often than not.
Shadow: I think the Lore Attribute is going to very rarely help you, which is a shame. The spells are pretty good all-around though. Not a useless spell among them, no matter what army you face (although Steed of Shadows is pretty weak).
Melkoth's Mystifying Miasma is one the weaker of the spells, but is certainly capable of providing benefit. Reducing WS is going to help against a lot of armies, reducing I or M will be useful against others. It's really going to depend on the army you face to determine which to use, of course. The Enfeebling Foe will be useful against almost every army, since it increases the survivability of your costly and fragile troops once they get into close combat. Watching an enemy need 6s to wound elves would be classic. The Withering will help a different way, by assisting your troops in killing foes. There's very few times that you'll find that spell pointless. Penumbral Pendulum is interesting, but I'm not sure how effective it is without giving it a shot. If your foes are coming to you, I could see it taking a toll on advancing troops. Pit of shades is fantastic, of course- you can use it combined with the Miasma to wipe out entire units. Okkamm's Mindrazor makes your units TERRIFYING. Strength 9 Black Guard? It’s another spell that will provide a lot of use. Just wait until an opponent is in combat with your crossbowmen enhanced by this.
This Lore makes a good choice for either a primary caster, or a secondary one. Some of the spells are a bit high on the casting values, however, so a level 2 may struggle to get them off.
Death: The Lore Attribute of this Lore partners well with Power of Darkness to keep your magic phase running longer. You won't get too too many dice with it, but even two or three dice are enough to get another spell off. Unfortunately, all but one of the spells that deals damage targets only a single enemy model (and not a unit), limiting the amount of wounds you're likely to cause. The spells here are short ranged, which Dark Elves can negate with the Focus Familiar. They are also great for sniping models out of units, which is actually pretty handy. I suspect the spells are too unreliable to see much use however. Purple Sun of Xereus is the big spell here, your level 4 has about a 90% chance of getting the spell, and it is very nasty, but the rest of your spells aren't going to be useful enough to make it worthwhile overall. I think this is probably the weakest choice for your Dark Elf sorceresses, but I may be willing to revise my opinion after a few games.
To start, I'm going to run my level 4 with Fire, and my level 2 with Metal. I will occasionally switch to Shadow on the level 4 to try it out though.