There are certain mass production modules that retain high values for various reasons. N4 simply did not enjoy heavy purchasing by individual gamers and therefore lacked large scale circulation.

Also, if memory serves it came out around that 2nd Edition transition period and several modules suffered "slow" sales and probably smaller print runs. The H series has some commonality here.

As far as content goes, it is an ok module. The 0 level aspect is cool, but that only goes so far.

I hope that sheds some light.