I agree with the DS1621+ over the DS1522+DS1621+, while a year older, gets you a better CPU, extra drive bay, and the ability to use a regular PCIe 10GbE NIC (get an Intel one, not the Synology branded ones) vs the expensive proprietary ones. I have multiple DS2419+, so I can't speak as to those two models specifically, but can give you a rundown as to what I do and don't like about Synology in general if that would be of interest. Software wise, they're really all the same.
I have plopped a ton of Synology into SMBs over the years (was even a partner with them for a bit). They never gave me issues and they were simple for me to remotely manage. That said - they are expensive for what you get. Your 4 PC backup and a couple VMs, needs are very small.Basically need this for a work to back up 4 computers....