my 6 rbps are in 4*2*2 tank abt 115 G tank, and they are also little bit more than 4 months old.
I bought them when they are 1 month old and still no problem, no cannibalism except some fin nipping........
try to keep the water clean and cool around 25 C. losing 5 fish in 3-4 months is not normal for me.
By the way when they'll grow old, they will be less aggressive towards each other