If you are using photoshop, after you select the Crop tool you will get some boxes in the toolbar that you can enter a ratio. Just keep it as 4x6 and you will be fine, but keep the resolution box empty or it will resample your image.
Picasa, free from Google has all the standard size ratios built in, just click the one you want and then the crop box holds to that ratio as you move or resize it. It's a nice little program and is very easy to use.
If you use Irfanview (as I always suggest that everyone do for viewing pictures), you can do to Edit, Create Custom Selection (or hit shift-C) and you can manually enter a pixel size, or choose from presets (640x480, etc), or change the option to inches and enter 4x6. If you want a 4x6 ratio to a specific pixel size, leave the selection on pixels, select 3:2 or 2:3 ratio on the right side of the box, and enter the pixel size you're after for either width or height, and it'll automatically calculate the correct number to keep that aspect ratio. Move the box around with the right button, resize with ctrl-left button, and when you're satisfied, hit ctrl-Y or go to Edit, Crop Selection to crop it. Hit S or File, Save As to save it as a new picture.
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