I think with the OWC 4M2 you really don't have to worry about that. Like I said, I've been using it without a fan for two years already and it works fine. OWC probably just chose that fan to save costs.I replaced a fan in another enclosure and found out the hard way that the fans that are used are calibrated a certain way, so the one I replaced don’t work and actually messed up the enclosure.
Besides that, every M1 slot only has a one lane PCI connection, so technically they're only running at 700 MB/s, which makes 2800 MB/s when you have four in (soft) RAID. That's also why I'm not too worried about running it fan-less.