Put a couple of droplights in the bilge with a coupel of 75 watt bulbs or just one with 100 watt or buy a small ceramic heater and set it at 70*. I have done all of the above with boats sitting outside in the driveway and also hanging in a slip. Never drained them and never a problem.
4 hours at 29* can ruin alot .
If there is water in your fresh water tank you may want another heat source in the cabin/cockpit area.