Two points first ...... You have 4 iPhones so I assume that you know that the phone number must be an iPhone number, FaceTime and Messages don't work with any other cell phone or smartphone. And FaceTime and Messages only work with other iOS users or Macs running mountain Lion.
Having said that - No - you do not have to have a phone number for these apps to work 100% of the time. Linking the phone number just came about in iOS 6 and the reason Apple did that is so that you can now sync the messages across all devices (including your Mac) and see your phone number as the contact at address for the apps.
I don't have an iPhone and neither does my wife and we Message or FaceTime and it has worked every time we have done. These apps work over WiFi and its not like sending an SMS text message with a cell phone.
My family shares an Apple ID and we have 4 devices set up to use FaceTime and Messages using different email addresses on all 4 devices. In my opinion, it is very easy to do and I don't see anything messy about it all. After all, most people have their own email addresses anyway. I figure just use my wife and daughter's email addresses as the contact at email address for FaceTime and Messages.
Frankly I don't see why everybody thinks that you have to have a separate Apple ID for this to work or it will get messy. It is no different than if I wanted to send my wife or daughter an email, they have to have separate email addresses for that. If you create an Apple ID for each of them, you are probably only using the existing email addresses that they already have, so what's the big deal about having to assign another email address for these two apps? I don't get it - but that's just me.
You can read this and see if it provides any help for linking your wife's phone number to messages on her iPad
http://support.apple.com/kb/HT5538
You can watch this about assigning different email addresses to FaceTime and Messages.
http://macmost.com/setting-up-multiple-ios-devices-for-messages-and-facetime.htm l