The Braille 'n Speak (BNS) is a device from the '80s that could be connected to a PC over its serial port and provide a TTS service for vision-impaired people.

Rasp 'n Speak aims to simulate such BNS synthesizer using a Raspberry Pi running emubns.

When the Rasp 'n Speak is connected to a DOS PC with a USB-to-Serial cable, it provides a Braille 'n Speak interface for screen readers. It means that a screen reader like PROVOX, ASAP or JAWS will be be able to detect a Braille 'n Speak device and produce actual speech through the Rasp 'n Speak.

I tested it by connecting a Raspberry 3 to the COM1 port of my 386 with PROVOX, worked fine for me. I did not make any extensive tests, so YMMV.
It uses espeak by default, but can be easily configured to use Piper instead, albeit Piper is somewhat slow on a RPi3. Hopefully it should work better on a RPi4.

More information on the project's website.
https://emubns.sourceforge.net/raspnspeak/