@wkuzma It's certainly not a terrible build, and the hardware would run Sims 4 on ultra graphics settings. But there are some potential issues with the build:
The recommended PSU for a Radeon 580 is 500W. 450W may be fine, especially if it's Corsair, but you'd be cutting it close. (I can get a second opinion on this is you want.)
ASRock motherboards are built fine, but their software support isn't great. If you or your partner is comfortable manually managing things like BIOS settings, it's probably fine, but someone who would like to not have to worry about these issues would be better off buying another brand (MSI, Asus, and Gigabyte are all good options). A B450M board is a good budget choice in general, with close to the same performance as the more expensive options.
If you get to select the parts, you might want to consider 16 GB RAM. It's not necessary for Sims 4, but it makes multitasking easier. Still, if cost is a factor, you can add more memory later—the motherboard has four slots, and it's usually not a problem to order more RAM, although you'll want to make sure it's exactly the same model as you already have.
256 GB storage is enough for Sims 4, saves and custom content, plus all the small apps you'd want. But it may not be enough if your partner wants to play other games, and it's definitely not enough to store large collections of music, photos, etc. Besides, the 512 GB version of the same SSD is only $23 more.
I personally would worry about the setup of the case. It's relatively small, so if this is your first build, it might be a challenge to maneuver in such a tight space. More importantly, it only has room for two fans, with just a single one pre-installed. I would guess this is probably not enough air circulation for a system running Sims 4 on ultra, especially since you're likely using an air cooler for the processor. The CPU and GPU fans can only keep the components as cool as the temperatures inside the case allows, and running a game like TS4 will generate a lot of heat in a short amount of time.
The overall idea of this build is good though—it's the minimum you'd want to comfortably run the game on ultra settings. A couple of small upgrades would make this a great build for TS4.
