There are always exceptions to everything..............but 8 pounds @ 4 months, is extremely unlikely for any breed.
30 pound bengals simply do not exist, unless you have an old obese cat. There is also no such thing as "bigger breed of bengals". No distinctions of size are made within the breed.
Adult female bengals average 7 to 12 pounds.
Adult male bengals average 10 to 17 pounds.
Occasionally you'll get a big boy, running close to 20 pounds, but is not the norm.
Lots of people and even breeders will exaggerate the weight of the cat, because they feel so heavy due to the compact size and muscular body. Bengals are built like army tanks.
My silver boy Jack is a big solid boy and when you pick him up he feels every bit like 20 pounds, but on the scale he's just 16 pounds.
Along with the untrue rumor that bengals are hard to handle and unpredictable because they have some wild leopard blood, we also have the rumor that they are huge cats.
The Asian Leopard Cats in the bengal lines is a very small forest dwelling wild cat that averages 8 pounds in size. The bengal cat isn't going to inherit any size from those ancestors!