What should you do if you used dot 3 instead of dot 4?

At the earliest convenous you should bleed out the dot 3 and change it out with dot 4. More than likely the dot 3 will be fine but the difference is the dot 4 has a higher boiling point for extreme conditions. So while the dot 3 is in the system no racing or towing and the system will be just fine. If you boil the dot3 fluid in your system the brakes will become very soft or squishy.