> why is it the Pegasos 2 G4 can't be passively cooled where some other system's G4s
> can? are there different types of G4s?

Of course, there are numerous types of G4 with numerous different max core frequencies and power consumption and heat dissipation figures each.

Refer to:

http://en.wikipedia.org/wiki/PowerPC_G4
http://www.freescale.com/webapp/sps/site/taxonomy.jsp?code=MPC8XXX7XXX (G4 has 7xxx designation (integrated G4 is 86xx), only currently manufactured ones listed)