With four drives, you likely won't need a 10Gbit nic. You won't be much over the ~110MB/s. 10Gbit nics run really hot, like GPU hot. I would recommend running it in 2.5Gbit mode, it is super cheap to get 2.5Gbit cards for your workstations and a 2.5Gbit switch with lots of ports is far easier to come by and cheaper. You also won't come near 10Gbit speeds with four drives.
I'd also recommend something like this over Synology. Synology uses very weak CPUS and everything is really sluggish. For a small office it is "fine" but if you want more bang for your buck. My server ended up costing me around $1,800 with 17 drives, 10Gbit, 800Mb/s, 16 xeon cores, and lights out. Granted it is all used, you could build something similar with a Ryzen 5600 or better with not a lot of money more.
I'll look into throttling it down. I got FCC money to upgrade our backbone to 10G. It's likely more than we will ever need, but I wanted to future proof since I will potentially be gone in a few years. I was looking into some of the other solutions, but for what we are doing this works pretty well and the software is easy enough that anyone should be able to come in behind me and figure it out after I retire. I've spent more money on stupider things. Thanks for the tips and advice! If I look into setting something up at home I will go your route.