rigid black 4" pipe underground about 15" impact resistant?
I'm in Minnesota (similar climate). It's been a fairly mild winter so far, but we did have one deep cold spell. All told, It was probably enough to freeze your soil sufficiently to bear that weight. The groove in your lawn is partly due to the low-density grass and topsoil, but it's hard to say without photos. 

That said, if your soil is very dry or sandy, it may have shifted enough to crack the pipe. At a temperature of 10 or 20 degrees F it won't be very flexible. 

One way to find out....