OK, let's crack this. We're given 870 and our task is to multiply it by 4. Essentially, we'll be adding 870 to itself 4 times.
Step 1: 0 + 870 = 870
Step 2: 870 + 870 = 1740
Step 3: 1740 + 870 = 2610
Step 4: 2610 + 870 = 3480

So, 870*4 = 3480