4 Wheel Drive or not

I've posted before about coming to a halt on a hill in a pass while towing because of police directing traffic around an accident. This was summer and there was morning dew on the bridge. As we were signalled to proceed I could see a semi loaded with rebar on the inside lane with his drive wheels spinning on the concrete. We started and my back tires went slowly sideways. Tried again and more sideways so pin weight was not helping. After my duh moment I switched the dial to 4wd and pulled easily up the hill. After that I knew always 4wd for me.