From what I understand about genetics, an F4B kitten would be a kitten whose mother was an F3, and Father was a savannah, but who had a grandparent that was not a savannah. For example, an F1 is always an "A" because it has a serval parent. An F2 can be an "A" or a "B", "A" if only its mother was a savannah, "B" if its mother and father were both Savannahs. An F3 can be an "A", a "B" or a "C""A" if only its mother was a savannah, "B" if its mother and father were both Savannahs, but one of its grandparents was not, or "C" if both its parents and all of its grandparents were savannahs. At the F4 level, kittens can begin to be SBT, which means that the closest non-savannah relative of the Kitten is its great-great-grandparent. Paige, et al please weigh in if I didn't get this quite right.
The differences between an F1 kitten vs. an F4 kitten. Neither is "better". It is a question of which would be better for you and your lifestyle. I don't have an F1, but we have an F2 little girl who is 8 months old, a 4 year old F3C boy, an almost 4 year old F4 SBT boy, and an almost five year old F5 SBT boy. Each of the kitties is different. F5 Diablo is the smartest and the most patient. F4 Mickey Blue Eyes is the most loving. F3 Jarvis is wacky, but wonderful. He took the longest to bond with me, so in some ways I think I appreciate him most. He sleeps with me almost every night, even though it took me more than a year for him to really let me touch him. F2 Baby Rey is our lap cat. If you are sitting down or laying down, she is usually snuggling with you, unless she is playing or eating. All of them are determined. All of them have different likes and dislikes. All of them are wonderful pets.