Should you worry about CP4 failure on a used diesel truck?

Yes, you should take CP4 risk seriously. But do not panic-buy or panic-walk without looking at the whole truck. The CP4 is a high-pressure fuel pump used on some modern diesels. When it fails badly, metal can contaminate the fuel system. Repairs can be expensive because injectors, lines, rails, and the tank may need attention. Before buying, ask for fuel-filter records, fuel-system repair history, recall history where applicable, and any signs of hard starts, low fuel pressure, rough running, or recent unexplained repairs. Be careful with trucks that have fuel contamination history or missing maintenance records. Some owners add disaster-prevention kits or CP3/DCR conversions depending on platform. Those can be good signs if installed correctly and documented. They are not a substitute for inspecting the whole truck. Price CP4 risk into the deal.