Actually a three wire solenoid is a little safer than the four wire solenoid for two reasons:
1) Forty years ago we used to hotwire Ford cars using two paper clips. One paper clip was fastened between the small right terminal and the hot battery side of the solenoid. The second paper clip was brushed against the two front terminals to crank the engine, which would then start.
2) You've probably read posts where guys make the outlandish claim
"I started my car, and the starter motor stayed engaged and even when I pulled the key out of the ignition both the starter motor kept running and the engine kept running... wtf was going on???!!!???" This is because the small right connector continues to receive 12v+ when the solenoid is engaged, thus bypassing the ignition switch circuit to the coil. So if the the solenoid sticks in the "engaged" position, it will also politely provide the ignition circuit its needed 12v+, so the car will keep running. The stock ERA wiring diagram actually lets this happen even if you have a MSD box wired in, and have never heard of, nor used, a ballast resistor in your life.
