The Catalyst 4912G switch (part number WS-C4912G) is a Service Category 12 item. So a Packaged SMARTnet offering for Category 12 is what you're looking for.
You just have to decide what kind of coverage you need: 8x5xNBD, 8x5x4, or 24x7x4; and on-site engineer or not.
CON-SNT-PKG12 Packaged SMARTnet 8x5xNBD -- Category 12
CON-SNTE-PKG12 Packaged SMARTnet 8x5x4--Category 12
CON-SNTP-PKG12 Packaged SMARTnet 24x7x4--Category 12
CON-OS-PKG12 Packaged SMARTnet Onsite 8x5xNBD--Category 12
CON-OSE-PKG12 Packaged SMARTnet Onsite 8x5x4--Category 12
CON-OSP-PKG12 Packaged SMARTnet Onsite 24x7x4 -- Category 12
