I COMPLETELY hear your cries for such a toggle. I too keep my phone on Edge for battery purposes, especially when I'm connected to wifi. I get a much longer battery life this way. I use an AT&T 4S on 6.1 and the 4G toggle in NCSettings works perfectly to change between HSPA+/3G and Edge. The interesting part is, on the AT&T 4S iPhones, back when iOS 5.0.1 came out (I think it was that firmware...it was somewhere around this time) AT&T asked Apple to remove the built in stock option in the settings app where a user could manually disable 3G and force Edge. They made this change because they are obviously trying to phase out Edge. But luckily with NCSettings, I am still able to turn Edge on and off freely via the Notification Center.
What phone are you using and what firmware? I am pretty sure this toggle only works on the AT&T models of the 4S.