How to "down sample" an jpeg file?

[splee]

I would like to change the resolution of a jpeg file, but keep the physical dimension the same, for example:
- original file: 800dpi, 4000x4000pix, 5"x5"
- converted file: 400dpi, 2000x2000, 5"x5"

How can I do this? Can someone pls show me how to do this using command line?

[fmw42]

see -resample at http://www.imagemagick.org/script/comma ... 1#resample