I'd recommend the Swarm because:
1. It'll be simpler to setup since you'll only need to properly set the volume, cutoff frequency and phase controls once on the included 1K watt class AB amp/control unit for all 4 subs, rather than individually for each sub with the other options.
2. It'll be much more aesthetically pleasing in your room than the other options. Each sub is relatively small at 12"wx14.5"dx28"h, is supported by 3 conical spiked metal footers, have the speaker wire terminals hidden by being located on the bottom and are positioned with the 10" driver facing, and about 1-2 inches away from, the wall. Once properly positioned, the subs look like attractive wooden pedestals in the wood of your choice. You can also utilize them as small end tables if you'd like.
3. It would probably be the least expensive option, unless you know of a seller who offers large discounts on REL subs.
Overall, four subs will also perform about twice as well as two subs in terms of bass power, speed, smoothness, dynamic impact as well as the levels of detail and seamless integration with your main speakers. I'm not certain but believe the 4x REL 812 option has the potential to marginally outperform the Swarm but would be more expensive and require the setting of the volume, cutoff frequency and phase controls individually for each sub.