This patch is pretty well but there is a little mistake for the flash programmation:
The lines:
# The information in the data sheet of April/2004 is wrong, this works:
         loadpage_lo     = "  0   1   0   0    0   0   0   0",
                           "  0   0   0   x    x   x   x   x",
                           "  x   x   x   0   a3  a2  a1  a0",
                           "  i   i   i   i    i   i   i   i";

# The information in the data sheet of April/2004 is wrong, this works:
         loadpage_hi     = "  0   1   0   0    1   0   0   0",
                           "  0   0   0   x    x   x   x   x",
                           "  x   x   x   0   a3  a2  a1  a0",
                           "  i   i   i   i    i   i   i   i";
must be replaced by:
# The information in the data sheet of April/2004 is wrong, this works:
         loadpage_lo     = "  0   1   0   0    0   0   0   0",
                           "  0   0   0   x    x   x   x   x",
                           "  x   x   x  a4   a3  a2  a1  a0",
                           "  i   i   i   i    i   i   i   i";

# The information in the data sheet of April/2004 is wrong, this works:
         loadpage_hi     = "  0   1   0   0    1   0   0   0",
                           "  0   0   0   x    x   x   x   x",
                           "  x   x   x  a4   a3  a2  a1  a0",
                           "  i   i   i   i    i   i   i   i";

I cloned the t2313 section to make a t4313 section, adjusting for the larger EEPROM and Flash sections.

Tested with "avrdude: Version 5.8cvs, compiled on Jan 15 2010 at 17:27:01" and an AVR ISP mkII.

I'm able to read and write the entire 4K of flash and the 256 bytes of EEPROM.

(The ATtiny4313 is hard to get in small quantities, but I found one source on eBay.)

Attachments:  (1) 4313.txt with the new section; and (2) DiffsFrom2313to4313.patch showing the differences between t2313 and t4313.