I'm trying to do my own IT consulting gigs and this bar owner wants me to help him with his sec cams.

I'm thinking get DVR software/hardware like a reolink box that backs up locally on the device and then makes a redundant copy thru FTP to a NAS ...

OR just go straight to the NAS as off-site backup.

Anyone have any experience with IP cams?

What kind of file sizes should I expect with 4k?

Does record on motion pretty much mean it won't record during off hours when there's no motion? That sounds nice...

Also, creating a file backup server using a backup program on Linux...

Like basically home made OneDrive, except you can do whatever directories you want and the only size limit is how much you're willing to spend on HDDs

I'm thinking get DVR software/hardware like a reolink box that backs up locally on the device and then makes a redundant copy thru FTP to a NAS ...

Depending on your infrastructure, you could use something like Blue Iris or ZoneMinder in a VM and use the NAS as direct storage. Make sure the NAS is a Raid 5, 6, or 10, so if a drive fails you can just swap the drive.

OR just go straight to the NAS as off-site backup.

Through Cloud or through a Off Site VPN location that you manage? If its cloud, just know you'll have to pay for storage monthly.

What kind of file sizes should I expect with 4k?

Large and it depends on how many cameras and how much history they want to keep.

Does record on motion pretty much mean it won't record during off hours when there's no motion? That sounds nice...

Yes, record on motion only records when it sees motion.

Also, creating a file backup server using a backup program on Linux...

Just use the NAS you have? If its large enough.