How do I get my belly button piercing to shrink back to normal?

As attractive as a belly button piercing can be, it can often cause problems later in some ladies should they become pregnant.  It will be difficult to specifically address your concerns here over the internet, especially without images, but I can make a few general comments.  It sounds like you had your piercing in for about 4 years before you took it out with your pregnancy.  Assuming you wore it for the entire time, or at least continuously in the beginning until the tract healed, it is likely that a tract formed which was completely lined with epithelium, or skin, and this is why the hole never heals even after the ring is removed.  Furthermore, depending upon your tissues and the forces imposed upon them by the pregnancy, the size of the piercing, and its position, the tissues around the belly button, which are already somewhat of a weak link in the chain so to speak, can become weaker, especially immediately adjacent to the piercing.  This in turn can cause preferential weakening and fracturing of the skin's elastic fibers near the piercing, creating these "V" shaped "scars" or stretch marks that extend upward through the piercing from the belly button.  Things won't ever shrink back to normal on their own because the fibers of the skin are permanently stretched or fractured in this area.  In these cases the only way that the appearance can be improved is to remove the affected skin surgically.  It would be best for you to see a board certified plastic surgeon with extensive experience in tummy tuck and belly button surgery for consultation to evaluate your problem and offer some solutions.  Best of luck.