The T400s uses a "Series 3" dock that is not compatible with previous versions. Specs for the 2 docks are annoying to find, and hard to compare.
Here's my summary table:
| Feature |
ThinkPad Mini Dock Series 3 (4337-10U) |
ThinkPad Mini Dock Plus Series 3 (4338-10U) |
| USB Ports |
6 |
6 |
Video
(any 2 can be used simultaneously) |
VGA, DVI-D, DP |
VGA, 2 x DVI-D, 2 x DP |
| Audio |
Analog microphone in, headphone out |
| e-SATA storage |
0 |
1 |
| RJ-45 Gigabit Ethernet |
1 |
1 |
Both have Kensington cable locks, ship with 90w adapter, 1yr warranty. The DVI-D ports DO NOT support the attachment of a DVI-D to VGA adapter.
Sources:
ThinkPad MiniDock Plus Series 3: http://www-307.ibm.com/pc/support/site.wss/MIGR-72873.html
ThinkPad MiniDock Series 3: http://www-307.ibm.com/pc/support/site.wss/document.do?lndocid=MIGR-72871