When smoking a 4 lb tri-tip at 225°F, you can expect it to take approximately 2 to 2.5 hours to cook. Note that cooking times depend on factors like the tri-tip's thickness, your smoker's specific features, and how cooked you want it. Use a meat thermometer to monitor the internal temperature of the tri tip. Aim for an internal temperature of around 130-135°F.  Remember, these are general estimates, and it's important to rely on the internal meat temperature rather than just on cooking time. A meat probe or thermometer will ensure your tri tip is cooked to your preferred doneness and achieves the desired tenderness and juiciness.