Because the right hand operand is converted to a string and the string representation of Array(4) is ,,,:
> Array(4).toString()
",,,"
If you use the array constructor function and pass a number, it sets the length of the array to that number. So you can say you have four empty indexes (same as [,,,]) and the default string representation of arrays is a comma-separated list of its elements:
> ['a','b','c'].toString()
"a,b,c"
How the comparison works is described in section 11.9.3 of the specification. There you will see (x == y):
8. If Type(x) is either String or Number and Type(y) is Object,
return the result of the comparison x == ToPrimitive(y).
(arrays are objects in JavaScript)
and if you follow the ToPrimitive method you will eventually find that it it calls toString.