Hmm - if you are only looking at Level 4s. Before I start, I would say that some armies are just inimicably linked to certain lores, but that is something that Empire has never really had to worry about - pretty much all of them are good, it just depends on your playstyle.
I wish there were some way we were able to get Loremaster on our non-special character wizards, or at least able to pick spells. I thought the Hurricanum/Luminark would be a good call here, but alas, it was not to be.
1) Light - makes up for our state troops having rather inadequate statlines. With Shem's and Banishment there is good shooting potential as well. The ability to make a coven of light, which Empire can do rather easily, should not be underestimated.
2) Heavens - because the two re-roll spells are really good. Curse is great versus poison and killing blow, the latter our Knights are very uncomfortable with. Harmonic Convergence is just an all-round great spell - everyone fears a roll of '1' and this spell allows you to re-roll them for a whole bunch of different tests. Iceshard Blizzard is actually pretty good if you take the time to use it propely - because -1 LD can really start to tip odds against people. Comet is just so useful for controlling the board, if your opponent hides his army behind a hill, make him pay for it. If he wants to come at you, make a minefield of comets between the two of you. The Lore attribute is great for dealing with flying chaff, as let's face it, our BS shooting doesn't like dealing with skirmishers! Also, if you don't understand the majesty of double helblaster harmonic convergence then try it out.
3) Shadow - It's not just the awesome power of Mindrazor, but Withering, Enfeebling and Miasma are also just great spells to have at your disposal. It gets number three because the casting values need a Level 4 really. Miasma has a nice low casting value, and if you use it intelligently, you can draw out dice very handily.
It's all a bit of a blur after that:
Death/Metal/Fire
Life/Beasts
Surprised at how much people are liking Life - I really don't care for it. It is a boring Lore, I personally feel it's pretty much hedging your bets as much as possible, which just doesn't appeal to me at all. For other armies it is acceptable, it's just not that interesting. Doesn't help you bloodily dismember/burn/explode your enemies in a suitably visceral way - therefore, not for me.