what does "setNew interpreter error: LOW_MEMORY,MEMORY" mean?

As @Wilberforce says...

Just to clarify though LOW_MEMORY,MEMORY means:

• LOW_MEMORY - Espruino ran low on memory while executing code, and had to delete command history and some other stuff so it could try and run your code properly
  
• MEMORY - Espruino totally ran out of memory and couldn't execute and more, so it had to stop running your code.
  
  

The code you actually want (using timeouts as intended) is probably:

function go() {
  digitalWrite(A4, 0);
  setTimeout(function() {
    digitalWrite(B4, 1);
    digitalWrite(B3, 0);
    setTimeout(function() {
      digitalWrite(A4, 1);
      digitalWrite(B4, 0);
      setTimeout(function() {
        digitalWrite(B3, 1);
        go();
      }, 20);
    }, 20);
  }, 20);
}

go();

Note that when you do it that way, you're actually able to do execute other code in the background as well, because you're not completely stopping execution.