I wanted to print something on my Taz 4 today, but I was greeted with an "Error: MINTEMP" message when I powered the printer up. I assumed it was simply down to my garage being too cold (it was somewhere around 20F as far as I know). I contacted Lulzbot support to see if there would be issues starting the printer up in these temperatures (after overriding the minimum temperature somehow) and they told me the issue was not related to temperature but meant there was a bad/broken connection somewhere.
Their support team was super helpful, especially considering this was a Sunday over a holiday weekend. I'm really happy that I purchased from them. Anyway, I followed their diagnostic process which involved bringing the extruder into the house to warm it up to room temperature and then measure the resistance of the thermistor and another component. Everything checked out OK and they recommended tracing the wiring back to the controller board to make sure everything was connected OK.
However, I connected the now-at-room-temperature extruder back onto the printer and everything started working again, so this really was an issue of the extruder just being too cold.
Although I got things working again, it wasn't much help as prints wouldn't stay adhered to the glass platform presumably due to the cold ambient temperature in my garage. It looks like I'll have to wait a couple days until the temperatures are back in the 50's and try again.
March 2016 Update
I purchased a small space heater for $25 from a local hardware store and have that inside the enclosure. On a cold day, I run the space heater for 15 minutes or so until the printer is warm enough to turn on and then I turn the space heater off. I am also now draping a blanket over the front of the enclosure to keep cold air out. Also, to prevent prints from peeling up at the edges, I applied a coating of UVA glue (a regular glue stick) to the bed when cold. I only had to do this once and now ABS sticks to the bed so well that I have to use a rubber mallet to remove parts!