Quick update, I replaced the stock cgminer binary on my C1 with ckolivas' s4 binary this morning (about 5 hours ago) and it has been running well since... with two C1's I've been averaging slightly above 2TH/s, with 3-5% rejects and has already found three shares.  Temps look good (running about 40C) and the only problems I've seen so far is the GH/s(5s) metric is missing and the discards are high as usual.

For comparison, the stock cgminer was running at around 1.9TH/s, with 5-7% rejects and I was finding one or two shares per day.  Given the timeframe, I'm not putting much weight behind the number of shares found, but there is clearly a significant improvement in hash rate and reject rate.   Temps are about the same.

Now, if someone would just find a block I could get paid too