At 4000 rpm the engine skips, what could be the problem?

I would guestimate that your throttle position sensor is going bad. Or vac line. Hard to say with out vehicle info

If you can actually hear the engine race up to 5000 RPM you may have a serious problem. But more then likely you cant actually hear the engine rev up. If this is the case I would suspect a bad Tachometer ground wire.

Bad Fuel injectors,replace them....................

If there is low compression in one cylinder the fault wil be shown more at IDLING as the engine will not be smooth. So there will be reduced power and it might also skip.
In th long run you will have reduced power of the engine even as the engine will not show on higher RPM and thus the fuel useage will be more.
It will be good to check out and estimate to get the compression corrected to all cylinders.

Are you sure what you found is the temperature sensor? It sounds like it could be a faulty camshaft position sensor. That would explain the erratic idle and rpm spike. Will aslo cause the engine to skip because of the irregular signal being sent back to the ecm.

My vg30 dnt rev more than. 2200

check whith scanner for codes,it coul be missfire,it need to be diagnostic

You could have a bad valvebody in your transmission or a bad pump in your transmission or your clutches are bad. In any case you have a bad transmission. Please rate my answer.

on the plugs that you changed did you notice a black carbon trace on the ceramic part of either one of the plugs,if there is this means the wire is firing to the ceramic part of the plug, you will need plug wires, if any mark on plugs you just installed replace them also

hello, i first must ask why that engine would EVER be reved to 4000 RPM? it is likely you are up against the rev-limiter and your ignition is made not to over rev. be very careful....4000 RPM on that engine could/will severly damage your tourque convertor........very expensive it is common for the EGR passages on that model to plug up with carbon, remove valve, clean ports, and retest for low speed miss/hesitation, good luck....marty