I use an old Brother HL-2270DW black and white laser printer.

To print on 6x4 cards, on macOS, you need to download Brother’s drivers from their support site. I couldn’t get this size to print right using the generic drivers that come with macOS.

Next, in the print dialog, add a new paper size for 4x6. I chose to use 0.25 inch margins.

To print a card, insert it into the front tray (i.e. the door with the Brother logo), such that the narrow 4-inch side is fed into the printer. When you open the print dialog, make sure to choose the 4x6 paper size, and set the orientation to portrait.