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>
This will crop the edges of 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.
