It partially depends on the application you want to run. I have tried running STARS on a Pi 3B+ using the Trunk Recorder application and found it doesn't quite have the horsepower needed. Even with the quad core processor and volk tuning, it tends to have frame drops while recording one call. It did a good job at decoding the control channel. I have not tried the Pi 4 yet, which may have more cpu power. I ended up moving my STARS receiver onto an HP T620 thin-client running Ubuntu 18.04. Those 620s are incredibly powerful, and can be purchased on ebay for around $40. I have been testing for two days now running the entire Virginia Beach P25 (
openmhz.com/system/vabeachva) on a T620, and so far it has done well up to a 7 simultaneous calls. Admittedly, I am a little skeptical it will handle that system when it is very busy. I have one other T620 thin client running Trunk Recorder that is monitoring 17 analog frequencies without any issues. One of the things I have found is it's best only use a sample rate that covers your system's channels, and no more. Excess sample rate is just wasted cpu over-head. And for SBCs and thin-client systems with limited CPU power that can be a killer. With Trunking Recorder I have found that 1024k is most efficient for my systems, although default is usually 2048k. Hope this helps.
