As a general rule for upgrading to cups 1.4 when something regarding
local usb printers doesn't work:

1. disable kernel usblp: CONFIG_USB_PRINTER=n
2. delete /etc/cups
3. (re-)install cups 1.4 with USE="usb"
4. configure printer(s) from scratch via the cups webinterface
5. if your printer is a multifunction device, be sure you got the udev rules
setting the device permissions in shape so cups can access the device

If for whatever reason (broken/old printer drivers) that doesn't work
out there's still the way to install CUPS 1.4 with USE="-usb" to get
the old CUPS 1.3 behaviour back.