I'm going to purchase a new 4-bay Synology NAS later this month, and I'd like some advice on which drives to fill it with.

Mark?

I've been using 4 WD Red drives. Pretty much identical to WD Green in terms of power specs, but rated for use in a NAS, according to marketing literature. So far, so good.

I have three green and one red. So far so good on all of them. I don't know if I'd notice any performance differences with other drives, since my NAS is pretty much cold storage at this point. I don't use it for as much as I could.

Dunno about reliability yet, but that new WD Red 8TB unit I just installed seems nice. WD claims it has "NASWare 3.0", which as near as I can tell is a marketing term for "suitable for use in small drive arrays". Which implies that it has TLER to keep RAIDs happy. 3yr warranty here too.

WD Green no longer exists -- got rebranded as Red.
Which is a Good Thing(tm) for RAID, since the Green label didn't enable TLER.

I, too, am using WD reds in my 2-bay Synology.

I had a weird failure where one of the two drives died, and the second one failed only barely after I'd replaced the first one and got the RAID pair re-synched. So I'm skeptical about WD drives and will be switching to HGST on the next failure replacement and/or upgrade.

That sounds like they were likely to be part of the same batch? Is it still valid to expect similar lifetime / failure modes in that case? Maybe I should simply buy a mixture of manufacturers?

As it is, I'm going to put some older disks I have lying around in it, so that I can play with the various disk configurations before I start trusting it with all of my data.

Interesting! Didn't know about that. I'll probably just get another red one if I replace those green drives.

The most discouraging thing about upgrading a device like the Synology is that you don't gain the space you're adding. I have 3 3TB and one 4TB drive. If I replaced a 3TB drive with a 6TB drive next (for around $235 according to Amazon right now), I'd only get another ~1TB of space I'd only see real advantages on the second drive replacement. Oh well, so it goes. So far I'm only at 5TB of the 8TB available, so I have a while.

I did the same, and just replaced as I went. It worked just fine.

Correction: The WD Green drives rebranded as Blue, with a trailing Z on the model number.
http://techreport.com/news/29251/western-digital-paints-its-green-hard-drives-blue-in-rebranding

Reason for wanting a Green/Blue drive: single drive, not part of RAID, so the drive should try harder to recover from errors.

Reason for wanting a Red drive: part of a RAID array, so the drive should give up sooner on errors, allowing the RAID to handle recovery.

This turns out to be remarkably difficult. If I search for NAS drives (on the assumption that they've got TLER), I end up with WD Red, WD Red Pro, Seagate NAS, and Seagate Enterprise NAS as my only options.

I'm considering buying 1 WD Red and 1 Seagate NAS from one distributor, then waiting for a couple of weeks (or using a different distributor) and doing the same again to see if I get a different batch...

Or am I being paranoid?

I bought three identical WD drives (I should have bought different ones, but I was i a hurry). They all came from Scan in the same order.

Judging by their warranty expiry dates on WD's website, two were manufactured in August and one was manufactured in February !