Sorry, not sure what you mean by very little detail? See this link. It clearly indicates that iPhone 4 and newer are eligible. This is a recycle program, where you send you phone in and you get a gift card. It is not for trade-ins:
http://www.apple.com/recycling/
You may be getting it confused with the upgrade program which has a very specific set of rules, and only applies to iPhone 6s and newer:
http://www.apple.com/shop/iphone/iphone-upgrade-program
Or with the trade-in Program:
http://www.apple.com/iphone/trade-in/?afid=p238|sGUlkP75s-dc_mtid_1870765e38482_ pcrid_117729693607_&cid=aos-us-kwg-iphone-slid-
It is a bit confusing, but both the recycle and trade in programs are very specific as to which devices are eligible.
The Trade-in program would only be valid for phones that are in working order. The Recycle program would apply to any device.
Cheers,
GB