Basically would it be feasible to use the Raspberry Pi 4 8 gb (mounted inside a Miuzei Raspberry Pi 4 Touchscreen case) in conjunction with an Arduino Uno r3 to operate a lcd display (ILI9341) for the furby's eye animation, a 1.5 in speaker (with a PAM8403 Amp Module) for the audio portion, and stepper motors for the ears and mouth, or is this just a pipe dream.
The Arduino would be inside the extracted furby boom skull with the wired in lcd display in the eye port, the speaker module into the side speaker grate, and the ear and mouth motors in the usual place. The Raspberry Pi 4 computer setup, with the appropriate power supply and Rasbian OS, would be on the back of the furby for Arduino IDE programing (version 1.8.19) and storing data for audio/video animation (for the eyes and speaker).
From what I've looked into, most people state that I'm better off using the Raspberry Pi to make any form of smart robot, but i wanted to get more intel on the capabilities of using both.
I honestly have no idea but yes probably it would work if you're determined enough. There's a lot to unpack there and I've come to find that on this forum, you really have to meet folks halfway. Nobody is going to research all that for you, or take the time and expense to try it. It's your invention, after all, right?
Start in stages. Anything is feasible, I mean they can reuse rockets these days and check out that Mark Rober and the squirrel ninja thing. I saw some folks on here hacking toy parrots, and Adafruit has lots of eye options (at least they used to) that may do what you're looking for without a whole Rasp Pi 4, that's pricey. https://learn.adafruit.com/animatronic-fizzgig-monsterm4sk-puppet
Ten years in this hobby (which is nothing) and you know what I learned? It's all about how bad you want that one of a kind device to happen. So, yes, anything's possible.
Did some digging and yes Raspberry Pi and Arduino can interface. In fact there are some people who have accomplished a companion robot build using both, so i just need to start from there and gradually explore the capabilities that both boards can accomplish together.