I recently purchased raspberry pi 4 (4GB) and I have three uses in mind for it. One, run Home Assistant (duh). Two, run a Unifi controller. Three, run a torrent box. It seems recommended to use the pi for only one task but the 4 has more oomph to it. Further, my initial Home Assistant setup will be pretty basic - a couple of lights and a couple of doors.
Its a little difficult to judge the necessary resources for Home Assistant since setups vary so widely. Any sage advice?
Don’t believe everything you read - try it yourself 
Once its up and running look at a system monitor such as htop and see how much RAM and CPU it uses. Right now my HA is using around 2% CPU and 10.5% memory, I only have 1GB RAM also.
My Pi3 handles running HA, nodered, amd a few other random things from github and it all runs OK for me, a Pi4 will be more than enough.
One other thing to keep in mind is what you can do with the RPi also depends on how you install HA.
If you install HA using the HassOS image then you can’t run anything else on the RPI unless the other thing you want to run is offered as an add-on.
If it’s not offered as an add-on then you will have to install some version of Linux on the RPI first and then follow the instructions to install Hassio on a generic Linux server.
If you don’t need (or want) to run Hassio then any other way of installing HA (that I know of) will allow you to have separate control over the OS to run other things alongside HA on the same machine as well.
If you want to use HASSIO there is a UniFi Controller add-on.
As for Torrent’s install the Portainer add-on and then use use any normal Docker Torrent Container.
In my experience, video is resource hog. Specifically transcoding/video processing but playback single vid no so bad. This may be even less on modern hardware but I have no base for comparison here.
If just running general apps on even Pi2 you can do a lot.
I am running HA, Torrenting, file sharing through samba, mqtt and some more that I cant remember now… All this on a single pi 3b+. I used to have motioneye too, but when I added a 3rd camera it had a hard time, so I just bought another pi and have motioneye on that. Video streaming or recording or anything like that, will make a difference on your pi, but I think it will be fine to run all other things at once.
I am sure the pi4 will be able to handle more. 
Hi, i got Rpi 3B, and i Do have running on it :
-Hassio
-Node red
-Mttq,SMB, all the basic Hassio components for a normal Esp8266 wifi sensor network no more than 10 nodes
You can run many things on a single Pi 4 but don’t put all your eggs in one basket.
You don’t want everything down if the hardware fails on you, and you don’t want certain resource intensive services in the same box in which you need good performance (like front facing stuff and essential services) like Hassio, Web Pages, proxies, etc.
Also you don’t want to freeze or slow the box in which you run your critical services (alarm, thermostat, locks, automations) or the interface you use to control all your home. You want a nice butter smooth experience and instant response times.
I learned from the first time my SD got corrupted.
Basically I was left without Unifi and PiHole (which were only on the Pi) and had loads of issues.
My network was unmanageable and my devices stopped working as the PiHole was my only DNS so they couldn’t resolve any addresses.
So now I have:
-RPi3B+ running Hassio (with Unifi, NGINX and PiHole which is turned off)
-ODroid XU4 running OpenMediaVault that hosts my Nextcloud(which I use to access my NAS remotely), PiHole (takes loads of ram after 2million domains in blocklist), PLEX, SMB, MotionEye, Print Server, Transmission. Also Unifi and NGINX (which are turned off). These services need way bigger bandwidth and processing so I need the gigabit connection in the XU4, also they were making my HomeAssistant very sluggish sometimes and making my backups huge.
My setup was done way before the Pi4 came out so there weren’t many options aside from a NUC. And I wanted something small that draws very little power and fits in my cabinet.
Aside from the need of having a local place for backups, my XU4 has a docker for Unifi/NGINX ready but turned off in case my Pi fails, so I can get up my network up and running quickly.
If my XU4 fails then I still can manage my network and change the DNS settings for all my clients (or just point the DNS server to my Pi and turn PiHole on).
Two days ago I just had my Hassio crash, no problemo: PiHole is still responsive so the clients can still resolve addresses (hosted on a separate box).
My NAS had the backups I needed so I was able to just flash Hassio to the microSD card and restore a backup within 20min. .
At the end it depends on your setup or what you want.
I’d grab that RPi4 and put some heavy stuff on it that will really utilize it and also to put some stuff to play with.
And move the HA and other essentials services to a box in which I wouldn’t touch much or that I know I’m not running sketchy stuff like Transmission, Nextcloud, PLEX, MotionEye which are intensive on the disk and network which may lead to slowness, unavailability of services, freezes (all within the same box) or even make your hard drive fail.
Just to chime in here. A Raspberry Pi can do many things. Even better if you can move most stuff off the SD card and onto an SSD.
The question is uptime/reliability. Unify and HA are probabyl your more critical components, maybe they can run happily together (with HA there is an addon for unify). I would question running a torrent box on the same device as the brains of my home.
If the Pi is the brains of your home, controlling your lights and locks and the like, whether it is Home Assistant or any other automation software, do you want to run other things and risk the brains of your home crashing? I run HA alone on the Pi3 and found it pretty usable, yet when I went to a Pi4 it was so much faster and responses in the UI and from voice assistants that took a second before became instant.
I wouldn’t want to run anything else on the pi and lose that that speed in anyway from my home automation hub.
Your milage will vary depending on what you run, but I am in a small unit. One door lock, 4 window openers, 6 switched lights and a couple of blinds. It’s nothing fancy along with about a dozen automations, but it really makes a difference if the processor speed isnt available.