Just a few more thoughts, AOG:
1. Have you tried printing a "portrait" orientation 6"x4" photo? The reason I ask is that when I use 6"x4" paper I always use the setting for 6x4 plus tab and this paper should be fed into the printer short side first.
2. Daft question, I know, but have you Saved the settings in the printer wizard before you print?
3. Have you tried the free image viewer / editor, Irfan View (www.irfanview.com)? It's well worth a look and it's fairly simple to get to grips with.
I have had a couple of HP printers - the first one expired after about 8 years of sterling service) - I now have a Deskjet F4180 all-in-one and while this is a little more complicated, even quirky, compared to my old one, it produces very good photos of whatever size, orientation and quality I want. I just have to remember to check the settings before I hit the Print button as Mrs XX sometimes likes to use different settings from mine. (The printer's memory is better than Mrs XX's!)