Is a gen iv 454 block the same as a LS6 block ???

yes

If your are referring to a MK IV 454, back in the day, they are either 2 bolt main or 4 bolt main. The LS6 like in a 70 Chevelle or Corvette are 4 bolt main, most all other production blocks were probably 2 bolt main, there could be some truck motors that were 4 bolt but I can think of any right off the top of my head. Now the newer style GEN V and GEN VI are all 4 bolt main.