This can be done using the Picasa software. But, in order to properly print your photos, you need to configure the appropriate settings on your printer, as well. Try going to Start > Control Panel > Devices and Printers, click on your active printer, go to settings and choose to print on 4x6 paper. In Picasa, you need to select the Print option on the full page. For more information, visit the following support page.
