So to defend against 4 u need to roll over a four. So if it's over 5 you cannot successfully defend because there is no 6 on the die.
I do remember in 40k 2nd Ed( that's what I played), to successfully roll a 7 on a d6 you first had to roll a 6 and then a 3+, for an 8 on a d6 a 6 then a 4+ Etc.
Anything like that in SHDA or are you just cattle trucked with large swarms?
Resort to special abilities like Counter Attack, Defensive Stance or Block. Otherwise you're dead.