There are various safety features built into the pcm to keep your engine from blowing.
P/N will not rev over 4K unless you are moving over 15mph.
3rd gear is possibly limited as well to a certain speed which would then kick in the fuel cut off (like park/neutral).
Oil Pressure: Won't effect the performance of the car because the PCM does not read or monitor the oil pressure.
4th gear: Technically it'* probably the 4th gear clutch hub that went bad. The splines are not hardened until the 2003 model year and the poor design strips out. I have a couple that I should take pictures of for reference.
00 Regal GS: All kinds of cool stuff.
If the oil pressure is pegged, 99% you have a bad sensor and it is unrelated to your llimited RPM issue. The loss of 4th gear may actually be directly related to the issue, in that the transmission failure may put the car into "limp home" mode (like Dan suggested).