In order to have copyright infringement you have to also suffer loss of some kind. This is also in the US copyright laws.
Yes, distributing copyrighted materials, even OOP, is by itself a violation of Copyright, but if you read the laws you will see where in order to bring it to court one of the 3 things you have to show is loss/damages.
Basically, if you cannot also show financial damages, its not worth going to court over, because its not worth the courts time to even rule on it.
Again, don't take my word for it, seek professional advice.
Lord Xeth, find and buy the Goodman DCC's made under copyright. They will be a good guide line to show you what you can and cannot get away with. I believe they said, "Compatible with the worlds favorite fantasy game." Or something similar. I don't think they used "4E" because I believe that might be copyrighted. If you have a 4E WOTC book look in the beginning, either on the title page or soon after. In small print they list what they have a copyright and Trademark on, which isn't much. Realize the copyright only refers to "artistic expression" stuff, so when they say the entire contents is copyrighted it primarily means the exact presentation, artwork, and layout design is protected. Not the rules, not the words, etc... They do have control of certain names, such as any specific characters or monsters.
However, this doesn't mean you can't write up a Red Dragon of your own. You just can't use their version, and the more drastically different you write it up, the better. Orcus isn't even copyright. Such names are used elsewhere in many texts. The Orcus WOTC has control over is the artistic picture they did up and the stats they wrote up for him. Plus the way in which they present/layout creatures in the MM. SO if you write up an Orcus, use an original drawing of him that does not copy anyone elses "Orcus", and write up attributes/powers that do not copy/mimic what WOTC wrote, then your fine. Oh, and do not use their same stat block format. Re arrange it.
I do believe they have copyright of "Mind Flayer" and "Beholder", but only when used in the same context of an RPG. Think about it. Read enough horror type stories and you will see "mind flayer", and everyone knows "Eye of the beholder", so the copyright only covers when the word is used as a name of a creature that also fits WOTC's description of a "Mind Flayer" or "Beholder".
However, it is still wise to avoid any conflict when you can. So even when you write him up differently you would still be better off to write it up not only with different powers, but a different physical description and call it a "Flayer of Minds".