Here's what the Lenovo support doc states: Detailed specifications - ThinkPad R400, T400.

From what I can find this system is limited to 8 GB total RAM (2 x 4 GB modules). I don't see any references on anyone trying to push this system beyond that.

Remember your OS is also a factor here, you'll need a copy of a 64bit version of Windows before even trying more than 3 GB.

As long as you can return the memory you could give it a try. If you do, first try one 8 GB module first then try two. Please let us know what you discover.