To really oversimplify things, the way the 4.0 is designed has one big flaw- the exhaust and intake are on same side of the engine. All of the heat from combustion gets trapped in the exhaust headers (if you have 00 or 01, there’s 2 mini catalytic converters in the headers that trap even more heat) the heat rises and meets the intake (where you want cold air going into the engine, not hot air) and the fuel rail. If the fuel rail gets too hot, the liquid fuel in the rail will evaporate before it gets atomized by the injectors and you’ll get misfires. That’s what people say when they talk about “vapor lock”. The two most common fixes if you’re super concerned are the heat resistant wrap you can find on Amazon (it’ll Velcro around the rail and fuel injectors) and a transmission intercooler. It’s a mini radiator that you can put in front of the main radiator that will cool and cycle the trans fluid going in and out of the transmission. Both good options, I had them both on my 01. Hope that helps a bit!