This is a strange question, but I have a trench with a water line (HDPE poly pipe) in it from my new well to my house that I haven't yet buried. There's a cold snap coming where we'll see temps drop to around 10F for 4 nights or so. The trench is filled with water (from profuse rainfall in the autumn) right now; it's a bit more than a foot deep. My question is, does the fact that the trench is filled with water protect the pipe from freezing at all? i.e. does the trench being filled with water have a similar effect to if the pipe had been buried?
The trench should have been dug deep enough in the first place that it's permanently below the frost line, and should be at least several feet deep in any case so I'm going to assume that this is the case.
Water and ice are not as good of an insulator as soil, but for four days it will be fine. It takes a good week of hard cold for ice to grow to even a foot of thickness on open water - something very familiar to people keen to go skating or ice fishing on open water - and your trench (I sure hope) should be much deeper than that.
If you're nervous it's easy to go out there and drill a hole - you'll see the ice will only grow by inches a day at best. Get it filled as soon as you can, though - with proper soil covering the trench it won't freeze at all, even in the coldest of winters. If it was going to be a week, not four days, and another ten degrees colder you might consider rolling some bubble wrap or placing some hard foam panels over the trench to add a bit of insulation but four days of 10F is not really enough to freeze that deep. The forecast for Seattle, as of right now, is also looking more like 20F for the next few days, not 10F (windchill doesn't count here) - and with a good amount of snow. I think you'll be fine.
If the pipe is at the bottom of the trench, the water above will protect it from freezing to some extent. You see the same effect in a shallow pond, where the surface freezes first and the lowest depths freeze last, enabling fish to survive. This occurs because water's greatest density occurs at 4 deg C or about 40 deg F.