I just had 30 ml of pepto bismol (bismuth subsalicylate) for an upset stomach. I realized that it expired almost 4 years ago. Is this something to worry about?

No. There is a 36 month expiration placed on all consumable products such as medications even though most are stable far longer as long as they're not exposed to hot temperatures. You will have no harm from using the pepto-bismol, but should get some fresh product when you can.