With the ever smaller direct drive extruders coming out, such as the Sherpa Mini, it might be possible to modify an existing IDEX printer to support Direct Drive DSEs, allowing the printer to print in any 4 materials at high speed, such as the ease of use of flexibles, or carbon fiber filled filament, without the hassle of dialing in retraction, or the cost that comes with automated multiple tool changers, and still retain zero wasted filament on purge shields or purge towers.
I currently own a Tenlog D3, modified to use 2 DSEs, and am able to print in 4 materials at once, but I am hampered by the use of bowden tube style of feeding the hotends, which makes dialing in expensive filament rather frustrating.
I humbly suggest someone far better at designing parts than myself, to modify the DSE carriage to install a pair of lightweight Sherpa or Orbiter extruders in a direct drive, or a direct remote extrusion option such as a pair of Zesty Nimble or a Fl3xeon remote direct extruders, or even with just a very very short bowden tube fashion) that attaches to the carriage, just above the extruders.
You could achieve the dream of 4 different filaments printed with maximum precision and speed, not having to wait for a tool head change, or using a Mosaic Pallet, who's waste block can be much heavier than the final part! Instead you produce little to no wasted filament with a printer that costs but a fraction of most quad tool changing printers out there.