Jay is correct. Series II is a roots style where the IV is a twins screw.
Roots vs. twinscrew has been discussed about 90 billion times. If you do a search, you are going to find a wealth of information.
Both setups use the same intake and intercooler.
Both setups use 30# injectors
The series IV uses an aluminum intake tube (tb) while the Series II uses a plastic one.
Both have heat exchangers and a water pump.
Both styles are pretty much bullet proof and will give long life.
BTW - You need to change oil on the series IV where the II is self contained.
Power output is low on both of them due to intake and intercooler design. It can be fixed but requires some cash to do it.