I'm not your lawyer, I'm not an IP specialist, and this isn't legal advice, but...
Generally speaking, names are not protected by copyright. That's what trademarks are for. So the first rule of thumb for "4e compatible" non-licensed works is don't use anything that WotC has asserted a trademark on. They have to do that openly (generally with one of those "beholders, illithid, X, Y, and Z are trademarks of Wizards of the Coast; A, B, and C are registered trademarks" text boxes near the beginning of the book, although just using the TM logo is enough, I think.) If eladrin is a WotC trademark (which I haven't checked), don't use it. Otherwise, you might be okay even though it's their name. Likewise, you're probably okay using the names of non-trademarked powers. The names themselves don't have copyright protection.
Also, any text in a specific form is copyrighted. You might be able to quote small sections as fair use, but I wouldn't if I were you-- the goal is to stay as safe as possible without a license, so I wouldn't want to rely on any nebulous doctrines like fair use for safety. So that means no copying text. And frankly, you probably don't want to copy stat blocks either, for the same reason. So, "3 kobold slingers" is probably fine (no trademark, no copyright protection for just a name), and 3 kobold whatevers, with a new stat block that you wrote from scratch, is also likely to be fine, but I wouldn't say 3 kobold slingers, and then copy out the MM stat block. Likewise, you can probably say "X is a warlock with the eldritch blast power" as long as (A) none of the names are trademarked and (B) you don't copy the text of the power description. There is definitely a fuzzy area here, but again, if your goal is to be on the safe side...
You'll also have to make your products not look like their products. There are "trade dress" issues to worry about there. But that's pretty easy as a matter of design to avoid.
My guess is that if you don't use any of their trademarks and don't copy any blocks of text, you'll be fine. Also, it's worth remembering that sueing isn't costless for them either. In fact, Hasbro would probably have to spend a substantial amount of money to sue-- that's both time for their in-house counsel office, and they would probably hire outside counsel at hundreds of dollars an hour. So sure, they can try to bully you out of existence by sueing, but they may not, unless they think it's worthwhile. If you make it easy for them (infringing their trademarks, republishing the PHB text for powers, etc.), or if you're really big, or if you have deep pockets, they may view it as worthwhile. But there is probably room for small fry to survive simply based on the idea that it's not worth spending tens of thousands of dollars litigating a claim over 1000 copies sold of a small-press module, especially where they might lose. (Remember, in the American system, each party generally pays its own lawyers, so even if WotC sues you for infringing their copyright and wins, they might end up losing money on the lawsuit. Conversely, of course, even if you win a lawsuit where they're sueing you for infringement, you're out your own lawyers' fees (unless their claim was frivolous, but that's a high standard.))
Anyway, I hope this is informative. Again, this isn't legal advice, but I wanted to correct some of the misinformation above.
