OK, let's crack this. We're given 400 and our task is to multiply it by 4. Essentially, we'll be adding 400 to itself 4 times.
Step 1: 0 + 400 = 400
Step 2: 400 + 400 = 800
Step 3: 800 + 400 = 1200
Step 4: 1200 + 400 = 1600

So, 400*4 = 1600