There are several cameras but they will not be as small as the SD400. AA batteries take more space than Lithium batteries.
The smallest, will be the cameras that use only two AA batteries.
Here is the list of 5 meg that use 2 AA batteries:
Fujifilm - E510, A350
Kodak - C340, C360
Nikon - 5600
Olympus - D595, FE110
Sony - P93, W1, W5
Here is a side-by-side look:
I like the Canon SD400, which is small and good enough for me. However, I need a camera that can use AA bateries since I may not be able find a place to charge the camera's battery when I am travelling. Any clue? Thanks