I am running a raspberry Pi 4 with raspbian, I am very new to all this, but have managed to install cast-web-api, which I am using to send messages to various speakers. My question is how, if at all possible, do I now install home assistant on the same Pi? All the instructions I can find refer to putting in another SD card and it looks like it formats the system. Any help would be good, cheers.
But note, if you do in fact just go down the route of shoving Home Assistant OS on an SD card and setting it up, it’s a better way to go, it’s better supported, and it natively lets you send messages to various cast sources anyway - so you aren’t losing any functionality.
You can follow this simplified guide also.
@mobile.andrew.jones @sheminasalam
OP says he is new to all this and you suggest him to install Home Assistant with the version for experienced users, which by the way doesn’t like any other software installed on the machine with the risk of getting an unsupported/unhealthy system 
I answered OPs question, but pointed him to actually using the Home Assistant OS, since it provides OP with the same functionality that they already get with cast-web-api