Short answer: No.
My trusty Mac mini served me well for years thanks to its 8 GB RAM and the 256 GB SSD. An SSD really helped make it faster back in the day. But with Xcode 9 and macOS 10.13 “High Sierra”, the machine begins to stutter.
I think it’s the CPU not really liking what I demand from it. Xcode 9 builds take a ton longer and the UI is less responsive. Sure, Xcode 9 sports a UI overhaul and might not be as performant as it could be, yet. That doesn’t help with the compile times which, for my current project, now take up to 5mins; that’s not even a fresh build.
So if you want to develop apps with Swift 4 and on the latest macOS, better get a more recent device. I don’t know which to recommend; the MacBook Pro is traditionally a good fit, but an iMac (or upcoming iMac Pro with its uncountable amount of CPUs and endless RAM) might be a good fit, too. Listen to Under the Radar ep. 89 for a more in-depth discussion by Marco Arment and David Smith.
I’d prefer a faster Mac mini because I have a nice-looking screen already and work at my desk 99.99% of the time anyway.