When sourcing a used JCB 4CX, you must prioritize the engine condition and hydraulic system integrity. Ensure the engine meets Tier 2 or Tier 3 emission standards depending on your local environmental regulations. Check the hydraulic pump pressure and look for signs of leaks in the boom, dipper, and bucket cylinders. Additionally, verify the transmission functionality, specifically the smooth shifting of the Powershift or Autoshift gearbox, and inspect the 4WD system and differential locks for operational consistency.