OK, let's crack this. We're given 405 and our task is to multiply it by 4. Essentially, we'll be adding 405 to itself 4 times.
Step 1: 0 + 405 = 405
Step 2: 405 + 405 = 810
Step 3: 810 + 405 = 1215
Step 4: 1215 + 405 = 1620

So, 405*4 = 1620