Siemens CardOS M4 smart card should work fine with OpenSC.
Currently only the Aladdin eToken PRO is tested often (a usb crypto dongle that contains a card with this operating system). It works fine, so all other smart cards with the same card operating system should work fine, too.
Siemens CardOS M4 does not allow a key to be used for signing and decryption. OpenSC has a workaround for this restriction, you can generate or store a private key with the "--split-key" flag which will store the key twice, with different usage options, but hide this detailt.
Some documentation is available from Aladdin for their eToken PRO, but for an in-depth documentation you need the Siemens card manual, which requires signing an NDA.
FIXME: where to buy such a card? pricing?