I haven't read of this being a common thing, there are many here who replaced their plugs after buying the vehicle with over 100,000 miles on their T4R and find out it had the original factory plugs in the engine, you can easily tell when they are the original plugs because Toyota installed Denso plugs in one side of the engine and NGK plugs in the other side, this is because the left and right heads come from different factory and they use the plugs supplied to them by Toyota in the heads and the factories were supplied with different brands of plugs, this is the same with the V6 and the V8 so there would be many reports of plugs becoming loose if this is a common issue.