Do not use the Print Preview to judge your image's quality the preview is a scaled view of you image not your actual image. Judge your images in Photoshop view at 100% zoom to see your image actual pixels. Also keep in mind the your Display's DPI is not your Print DPI the image on your display will most likely be larger and less sharp then your print will be. Make sure you have a goon number of Pixels to print a high resolution image. For a 4" x 6" 300DPI image you need to have at least 1200px by 1800px high quality pixels. Print Resolution is the print pixel size.
You cans have more Pixels. Printers can print pixels any size up to theists max resolution. The printers DPI setting is a quality setting not the images resolution. If you have 2400px by 3600px printing at a 600DPI resolution would produce the same 4" by 6" print to tour eyes because your eyes can not resolve down to the print pixels size of 1/600 x 1/600 of an inch 360,000 pixels per inch you can resolve down to 300 dpi 90,000 pixel per inch.