Look at the cost per GB. If you can get a 18TB drive for under $0.20/GB vs a 4TB at say $0.30 per GB it would make more sense to get the 18TB drive and also use it as a NAS to offload some of your data you're keeping locally. I keep most of my data on my server / NAS instead of on my laptop at this point as I tend to wipe the drives every 6 months or so to completely refresh the OS as they release new versions.
Since you're talking about 4TB of space you could go NVME / SATA M2's and bump the speeds considerably. At roughly $100/TB it's a bit more expensive but, it's also 5 times faster and more durable than a spinning drive. It depends on how you want to do things and personally building a PC as a NAS makes more sense if you want flexibility and performance. By using Linux and 5 drives in a Radi10 I get 400MB/s+ and a hot standby drive in the array that takes over w/o any intervention.