And 5x7? And 3.5x5, which was the standard before 4x6 came along.
Looking through the new B&H catalog I see that Canon, HP, Epson,
Ilford, Inkpress and Moab sell 5x7 photo paper. Museo has several
unusual paper sizes (they include matching envelopes) - 4.5x5,
4.5x6, 5.5x7.5 and 3.8x9.1 for panoramas. Epson also has a
heavyweight 4.1x5.8 size photo paper that's printable on both sides,
used for postcards and invitations. HP has an odd 4x12 photo paper
that might delight photo stitchers for their landscape and
architectural shots.
See below for 3.5x5.
In addition to their dedicated 4x6 printers, Hi-Touch has several
affordable 700 series dye-sub models that print three sizes, 4x6,
5x7 and 6x8. B&H sells these as well as the paper and YMCO ribbon
kits that they use.
There are also a number of much more expensive printers not
intended for home use in B&H's catalog. Prices are $1550 (Sony,
3.5x5, 4x6, 5x7), $2650 (Kodak, 4x6, 5x7, 6x8), $2996 (Kodak, 8x10,
8x12), $2750 (Fuji, 4x6, 5x7, 6x8, 6x9), $3995 (8x10, 8x12).