[Halon]

Yest there is. It pretty much just means it also uses a synthetic base oil other that PAO's which are what Group 4's are. I honestly do not know the advantages or disadvantages between the 2.

Here this is also taken directly from Amsoil's site.
Group IV are polyalphaolefins (PAO) which are chemically engineered
synthesized base stocks. PAOs offer excellent stability and molecular
uniformity and hence improve performance. Products labeled as "100%
Synthetic" use only Group IV base stocks and therefore are free of
petroleum oil and the impurities they contain. AMSOIL uses only Group IV
base stocks and is thus a 100% Synthetic motor oil.

Group V base oils, with few exceptions, are also chemically engineered stocks
that do not fall into any of the categories previously mentioned. Typical
examples of group V stocks are Esters, polyglycols and silicone. As with group
IV stocks, group V tend to offer performance advantages over groups I - III.
An example of a mineral based group V exception would be a white oil.