maximum rev?

I own a used 01 and have a question about reving the engine. I was reving it and realized that it wasn't getting past 4000 rpms. It would get up to 4000 and then decrease to 3000. I checked my accelerator but it was holding in the same position. I was wondering if this is an internal safety feature or if something was just very wrong with my car. Any suggstions?
p.s.- i'm just getting into cars so excuse any un-technical vocabulary, also sorry if this is a dumb question.

pcm limits revs in neutral to protect engine.

I suggest you do a pcm upgrade,I find that this along with cold air intake allows this glorious powerplant to breath!