You want all your photos to be 6x4 with a width of 1024, right? That means they should be 683 pixels high.
If that is correct, what you're looking for is ...
convert <input_image> -resize 1024x683^ -gravity center -extent 1024x683 <output_image>
... where you would replace with the filename of the image you want to resize, and with the new filename.
This will crop the edges to fit the aspect and resize them to 1024x683. It will make small images larger, as well as making large images smaller to fit your size.
When you want to resize all images in a directory, you'll need to write a quick bash script, which would look like ...
for f in *.[jJ][pP][gG]
convert $f -resize 1024x683^ -gravity center -extent 1024x683 print_$f
You then just make the script executable. (Say you saved the script as rename.sh, you run
chmod u+x rename.sh.) Then you run the script
./rename.sh which will resize all the images in the same directory.