If $n$ is a positive multiple of 2, the notation $n!!!$ represents the product of all the positive integers up to $n$ that are divisible by 2. For example, $8!!!=2\cdot 4\cdot 6\cdot 8=384$. What is the units digit of the following sum?
$2!!!+4!!!+6!!!+\cdots +1998!!!+2000!!!$