Android Gradle: What is javaMaxHeapSize "4g"?

This is an undocumented option to increase the heap size for dex operation: https://groups.google.com/d/msg/adt-dev/P_TLBTyFWVY/4TPJ2YY6khUJ

As it mentioned in the answer above, it is just an option to specify the maximum memory allocation pool for a Java Virtual Machine (JVM) for dex operation. And it's the same, as to provide to java the -xmx argument. Due to it's source codes from here, it's setter look like:

if (theJavaMaxHeapSize.matches("\\d+[kKmMgGtT]?")) {
    javaMaxHeapSize = theJavaMaxHeapSize
} else {
    throw new IllegalArgumentException(
            "Invalid max heap size DexOption. See `man java` for valid -Xmx arguments.")
}

So, you can see, that the accepted value should match the \d+[kKmMgGtT]? pattern, and hence not, it even refers to the man java to get to know, how to set the -xmx. You can read the man page here. And it says, that this flag:

Specify the maximum size, in bytes, of the memory allocation pool. This value must a multiple of 1024 greater than 2MB. Append the letter k or K to indicate kilobytes, or m or M to indicate megabytes. The default value is chosen at runtime based on system configuration.

In your example, 4g is 4 Gigabytes and this is a maximum heap size for dex operation.