@slegsonly 40Mbps per apartment on average...
You'd be fine with the 4100, but I would probably go 6100 so you can grow your ISP speeds in the future.
We don't charge for performance upgrades after purchase - that's just something the other guys may do. For future-proofing I'd get the 6100. If you're going to do logging get the MAX.
You can also email sales@netgate.com and have one of our sales reps work with you.