obd0 code 4 is your Crank Angle Sensor... which is located in the distributor. There are two sensors in there. One is your Ignition Output sensor... which will have a blue wire going to it (off of the small connector on the dist.) And the other one (should have three slide connectors) is the crank angle. They take about ten-fifteen minutes to replace and aren't expensive. Although your best bet is to find one at a junkyard. I don't think Advance auto or anyone carries them anymore.

obd0 CAS' are notorious for going bad, and this would def. engage the artificial rev limiter you're seeing while in "limp mode."

-Check to make sure all your connections are night and tight on there (should be 3 slide-on types)
-if the connections are tight, and you don't see an excessive amount of corrosion; then it's def. time to change it out. (which is a piece of cake)

Good luck man
-jonathan