I've already replaced the battery, and I do have the tools. The one I put in supposedly has nearly twice the capacity of a stock one, but I didn't run a charge/discharge test to measure it, so it may be crap.
Sounds par for the course with apple not allowing a downgrade path. That's one thing I really like about my Opera browser; I think they're up to v24 but they still support v12. I genuinely hate every version after 12, and moreso with each one, but 12.17 is teh awesome. I seriously used to get 3-4 days out of this 4S with 6.whatever and the stock battery, and it went down to <8 hours when I updated to the first ios7 version. It'll go a day now, but not much more. This is the first apple product I've ever owned new (only because it was a $.99 upgrade), and it will probably be the last.
When you were getting 3-4 days out of your 4S, were you doing anything with it other than making the occasional phone call and/or sending a text or 2? I've never gotten more than a few hours out of mine unless I pretty much used it as a paper weight. I'll admit that if I don't have something to do or just get bored, I'll surf the net on mine -- a lot. (which would explain the reduced battery life).