Recently I bought a Raspberry Pi 4 and also bought a hardware kit from Zynthian, an open-source music synthesizer project:
https://www.zynthian.org
Zynthian includes synthesizer software, running on a Linux distribution, and they also sell hardware kits for Raspberry Pi that make it into a small box with audio and MIDI ports and an LCD screen:
https://shop.zynthian.org

I just got the Zynthian kit a few days ago and put it together. It was fairly easy to put together (it took me about 2 hours). The Zynthian software was also easy to set up - They have an image file that you can write to a micro SD card, and it's all set up to run. You just need a MIDI controller keyboard and speakers to plug into the Zynthian. Zynthian is fairly versatile too. I haven't explored it much yet but I think it's pretty cool.

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