I just received my Lightroom 4 upgrade and what a surprise – after the installation all my Nik Software plugins dind’t work.
I check on Nik’s website and downloaded the latest versions, which is stated to support Lightroom 4, but the result was the same. The only filter that is working correctly is HDR Efex Pro. For all the rest under “Edit In” menu I get this message:

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Unexpected error performing command:
bad argument #1 to ‘lower’ (string expected, got nil)
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The plugins DO work with the Lightroom 4 beta, but not with the original version 4.
I sent an e-mail to Nik’s support – still waiting for answer. I’ll keep you posted of how the problem is developing.
I believe this is a situation that all of the current Lightroom and Nik’s users will experience these days after they upgrade to LR4.