I can answer my own question because it wasn't the heater core that was at fault. 4.1 - 4.3 ohms is the correct reading for a working 40W heater cartridge for the MK6+.
The problem ended up being a melted connection on the MK6+ control board. At one point during operation the wires that go to the safety shutoff circuit from the control board got snagged by the X-Y carriage. This snaggage caused a wire to be pulled almost all the way out of the control board, but not quite. Maybe only 1 or 2 strands of wire were left engaged. As time went on those few remaining wires got really hot and melted the plastic around that positive connector. Eventually the few wires left just got oxidized / corroded to the point where they couldn't carry any amount of power.
The reason why I thought it was the actual heater element is because when I measured the voltage coming out of the control board and out of the safety shutoff circuit I got ~12 Volts so I thought all good was there. I decided as one final test to hook up a small DC motor up to the leads and when I turned the heater on the motor didn't spin, that's when I decided to crack up the Thing-O-Matic and look at the connections. That's when I noticed the burnt / melted connection.
Since I couldn't even unscrew the lead due to the plastic melting I ended up soldering to the pin from the underside of the board.
