At 300dpi, you need an optimum res of 1800x1200 pixels, 1024x768 should do fine but not very optimum (as it's enough to print at that size at 150dpi or so).
Note that 1024x768 is in 4:3 ratio, it will be cropped or printed within the 6"x 4" (4R size) frame.
Here's a chart for your ref:
link