Interesting. I'm not qualified but have done a lot of research in this area. There are three assumptions with this method of dating:
When the rock forms (hardens) there should only be parent radioactive atoms in the rock and no daughter...
4150 years, with a margin of error of 300 years plus or minus, is based on radiocarbon dating of wood buried beneath the ejecta. I'm certainly not qualified to challenge that finding. Are you?