For Fast Mode printers with a single tray holding A4 media, PPDE can be used to configure an extra (virtual) IPP media-source entry for Letter, that maps to the same A4 tray. The two media sources will be visible in the Proxy Printer Edit dialog, so A4 and Letter media size can be assigned to them, and both A4 and Letter jobs will be Fast Print candidates. With an SPExtra rule the Letter media is substituted with A4. And, since fit-to-page print-scaling is applied for Fast Mode printing by default, the Letter job will correctly scale to A4.

The .ppde of the Fast Mode printer looks like this:

#-------------------------------------------------------------------------
# Two trays mapped to a single media-source holding A4 media. 
# In SavaPage Admin Web App: assign A4 to tray-1 and US Letter to tray-2. 
# ------------------------------------------------------------------------
*VENInputSlot media-source
*VENInputSlot *Auto tray-1
*VENInputSlot  Auto tray-2

# ------------------------------------------------------------------------
# When job is US Letter, send as A4. The fit-to-page strategy 
# for Fast Print will make Letter scale to A4.
# ------------------------------------------------------------------------
*SPExtra/media/na_letter_8.5x11in: letter-to-a4 \
   *media/iso_a4_210x297mm