Q: Motor revs to 4000 rpms until it reaches 50 kpm then drop but idles perfect in park, what could cause this?

Is the actual engine speed reaching 4,000 RPM or is the RPM gauge simply misreporting the engine speed? If the gauge is misreporting the speed, that is likely an electrical malfunction and to resolve it you can request an electrical circuit/component diagnostic. If, though, you are experiencing an uncommanded "excursion" to high RPM’s, that could be caused by a number of faults. In the majority of vehicles, engine idle speed is controlled by an idle control valve (ICV). The ICV valve can fail or a faulty or blown fuse in other electronically controlled systems can cause the IAC motor to malfunction, leading to a higher than normal engine speed. Failures in the ICV valve itself are often due to excessive carbon buildup in the intake manifold. Other possible causes of a uncommanded high speed include vacuum leaks, a malfunctioning throttle, cruise control malfunctions, and, rarely, a PCM/ECM malfunction. With regard to vacuum leaks, on a fuel injected engine a vacuum leak will cause a high engine idle as the engine’s oxygen sensor detects the extra oxygen entering the engine through the vacuum leak. The newly lean (too much air relative to fuel) status will result in the engine’s computer pumping in additional fuel, resulting in an abnormally high idle. A malfunctioning throttle system can cause both a high idle as well as an engine stall. A dirty air intake or cracked air intake tube is often the culprit. If you have engine speed "excursions" and you desire that this problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an engine idle speed diagnostic - high idle and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.