Crucial have a compatibility tool that will list modules with guaranteed compatibility - from that it looks like the T420s only supports 8GB total as 2x4GB

DDR3 PC3-12800, DDR3 PC3-10600, DDR3 PC3-14900

Memory Type: DDR3 PC3-12800, DDR3 PC3-10600, DDR3 PC3-14900, DDR3 (non-ECC)

Maximum Memory: 8GB

Slots: 2

I would double check with Lenovo though