Well, firstly, if the water change was 4 days ago, I would expect your tanks to read 0 ammonia, because that is what the filter bacteria is supposed to do. The bacteria got more food than they normally do, but the colony size grew and took care of that extra ammonia, probably pretty fast. If you did a water change with 1 ppm ammonia water, even if it was a 50% change, I'd bet that your tank would read 0 ammonia in a matter of hours, no more than 8 or so. All that assumes the tank was fully cycled before the water change, of course.
Now, I'd keep an eye on the ammonia levels in your drinking water for a while. When there is a lot of rain/flooding, the water company does add extra chlorination to help kill more bugs than usual -- and since most use clhoramines, they may have accidentally used a little too much ammonia compared to the chlorine. The other possibility is that Iowa is pretty agricultural area and ammonia is a common ingredient in fertilizers, so the run off from the fields may be the source of that extra ammonia.
Either way, you do want to monitor the levels because you really shouldn't be drinking ammonia in any concentration. For a few days, a low concentration is okay -- your body can metabolize it into something harmless. But, long-term exposure can have heath repercussions -- softened bones for example. Your water company will want to know about this, if it doesn't clear up in a short time. Sometimes it is caused by more serious issues, like leaks in the pipes or something like that.
Your fish are actually probably just fine using that water -- again, the filter converts it pretty quickly really.
All that said, if you are going to be in the neighborhood, I would take a sample to your LFS and have then test the water, too, just to make sure that your test kit is still accurate.
