I resolved this, but want to document it for others, and ask if anyone else has seen this.
I had issues when I first hooked up the 4A. I bought two of them, and they both had the same issue. I was using a small DC motor that came with my Arduino UNO kit. I used the motor demo code to test. I also tried some of my own test code that I had used before with the L298N controller.
The UNO was powered by its USB.
The motor would run as expected for a bit, but at each direction change the red LED on the 4A would flash. Within a minute the red LED would stay on and the motor would stop. I read all the docs I could find on 4A and its chip, but couldn’t find info on the LED warnings. This happened with the demo code and my own proven test code. I had originally joined the 4A on top of the Arduino, joining all the pins. I tried separating them and using jumpers, but it didn’t change anything.
Finally, I removed the USB cable and hooked the 4A up to my variable DC power supply, and set the voltage to 7V. At this setting, the little DC motor ran perfectly from then on, without issues.
I then switched to an 18V DC motor, and it worked fine as well. And when I ran two 18V DC motors simultaneously, there were no issues.
So I’m guessing that the 5V the 4A was getting from Arduino was insufficient to even run the little DC test motor. Note that I have run the same little motor with various L298N boards with no issues.