Other replies have given you good advice to use as a jumping off point but assuming you are taking data you find very meaningful (pictures and videos) and putting it on hardware you own there are two concepts you should familiarize yourself with asap before going too deep into purchasing stuff and building/installing

RAID: provides redundancy for when a drive inevitably fails. Will require multiple drives to create an array (the amount of drives depends on what kind of raid) but this helps you avoid downtime. Drives will fail, it happens.

Backup: provides resiliency for when your raid array entirely fails so your data isn’t completely destroyed. As you learn about and experiment with raid you may be lulled into a false sense of security, especially the first time you resliver a drive into the array. It’s honestly a bit like magic, you pull the dead/dying drive out and pop in a fresh drive, test it to make sure it’s good to go (not essential but good practice), and then after awhile it’s rebuilt from parity and your data is back. But this process is stressful on the drives and after years of running a home server I’ve finally had an occasion where reslivering caused an additional drive to fail.

Additionally all of the above is pointless in a disaster situation. If there’s some kind of extreme hardware failure that zaps all your drives at once, your house floods/burns down/etc, raid won’t help you obviously at all.

This is where backup gets annoying. If you have 4tb of data it’s pretty inexpensive to create a local backup, a 4-8tb external hard drive (or even larger) can be purchased or made for a couple hundred bucks. Sometimes you can get a refurb 18tb drive under $200 and slap it in a cheap external enclosure.

But offsite for that amount of data is costly and pulls you back into the amazon/google/apple/etc ecosystem, which if you’re anything like me you’re trying to get away from. Another option besides an external drive is a bluray burner and discs. They’re relatively cheap now. I have a different off site backup solution now that I was able to purchase property but the bluray thing was what I did prior to buying land. The downside is that backups were done at longer fixed intervals so there was a risk of losing ~3 months of data. I would burn them and then take them offsite, to a trusted location that I visited fairly regularly.