With your data requirements, a 4-bay is fine. You can fill it with 4Tb HDD in a RAID10 for performance giving you 8Tb usable. Plenty for now and the future. If down the road you need more space, you can swap them out with 8Tb or better units. Even 10Tb HDDs drives are very reasonable price wise.
Make sure you have a good backup system. Failure of the unit is your biggest liability. It is a single-point-of-failure. The "RP" models make redundant the most common point of failure aside from the drives themselves, but losing the chassis is still a possibility. We use NAS's like this for "non-critical" data like archives. If the building burned down and we lost them, we'd grumble, but nothing in them is critical for the business to run. It's "luxury" data.
As mentioned, the + models offer replication so you can just buy a second unit. If the business is OK with the unit being down for a day or two and you have backups, you could just order up a new unit in a pinch and move everything to it. If not, then a second unit is a viable option. Regardless, MAKE SURE YOU HAVE BACKUPS since Ransomware will encrypt a whole replicated system without a second thought, and then you have two NASs filled with garbage.