Is possible especify the keys rather how many in multisignature wallets?
You are right, multisig as implemented in Bitcoin right now requires M participants out of N to sign. However, the use case you want is doable with more complex scripting.

For example, K1 + (K2 or K3 or K4) could be done with something like:

&lt;pk1&gt; CHECKSIGVERIFY 1 &lt;pk2&gt; &lt;pk3&gt; &lt;pk4&gt; 3 CHECKMULTISIG

That script can be redeemed with the signature of K1 and a signature of one of K2, K3 or K4.