Using serial values from an Arduino to trigger outputs on a Raspberry Pi?

I know this is kind of advanced and I honestly might be in over my head with this, but here's what I'm trying to do.

I have 4 analog sensors hooked up to an Arduino Uno. If the sensors read a value between 20 & 50 I'd like them to signal my Raspberry Pi and trigger an output. I am hoping to do all this through USB/serial communication as I've been having nonstop issues with my level shifter. Any suggestion?

cross posted.