Wouldn't say that I specialise in this kind of thing, buy over the years I have more than a passing professional and personal interest...
I've been through all the pain of many solutions, including a PC based file server, which while quick and easy to get going chews through 'leccy like no business. Most of the kit doesn't have to be expensive, however if you aren't careful it will get expensive very quickly.
You don't need gigabit ethernet for streaming, and as noted above, most of the kit just doesn't do it anyway. Wifi is a waste of time for streaming, while the headline speeds may seem good, the real speeds are never close and as soon as you get multiple devices connected (or neighbours with wifi) the performance rapidly drops below useful levels.
NAS: Currently I'm using a ZyXEL unit of some form, with pretty much most features turned off. Stick to the basics, ignore the "value added" functions and most commodity NAS devices will do the job. I chose this ZyXEL device because of its support for Linux and Windows sharing, and the reviews reported success with both (and it powers down to low power usage). It's a single 2Tb drive, while RAID of some measure might sound like a good idea, it's not usually useful and adds needless complications and power problems (it's not that I'm not a fan of RAID, just sometimes it's not always useful). Redundancy, backup? Easy? Buy a second NAS and that's your backup. If you're careful and upgrade regularly you can buy a new NAS device with extra capacity, copy the old content onto it and have an instant backup of when you last had the content. Unless you throw your devices about and are willing to replace a HDD drive every couple of years, you are considerably more likely to lose content through accidental deletion than HDD failure. If you're particularly paranoid, power the NAS through a reasonable UPS so you get both power level smoothing and a few minutes of battery backed up power (I do).
Playback: It's generally best to have a single playback system for each screen. This gives you maximum redundancy but also keeps the cabling and communications sane. While you could have a super playback system playing four independent streams, you will quickly suffer internal bandwidth issues but more importantly you have to both stream the video and audio content from this one box to each screen and feed IR remote control signals back the same distance. The further the distance the nastier both of these become. I'm currently using an Acer Revo box with a Microsoft media centre IR controller connected for each screen. These PCs are cheap (£200-£250), small and are pretty low power particularly if you fit an SSD or low power HDD to them. Many are effectively silent or fanless as well. On the software front I get the cheaper Revo devices that don't have Windows on them, wipe the junk they do come with and install a build of XBMC on them. In the past I've had annoyances with audio playback but these days the drivers are all just there and the Linux audio layers have matured sensibly.
Security: Fit everything into your own wired network. You can add usernames and passwords for sharing files and for a home system you really shouldn't need much more. XBMC supports content levels so you can protect minors from inappropriate content.