We need some more information... Technically any printer can print at 4x6. There are specific printers that only print on 4x6 paper, but I normally just print multiple photos on one sheet and cut them out.
I don't know what program you use, but a BIG factor in the print quality is the DPI. I always make new images at 300dpi and paste the picture into the image. I then tweak/free transform it so it fits correctly. Doing it this way almost always results in a much higher quality print than say using a more standard 72dpi template.
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Primary- i7 920 | 6GB RAM | 9800GT | 160+160+500GB
Secondary- Athlon X2 4200 | 2GB RAM | 7600GT | 80+250GB
Netbook - Atom N280 | 2GB RAM | GMA945| 160GB
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