Dear Student ,
P = RI2 = 10*1 = 10 watt each resistor
P2 = R2I4= 5*42 = 5*16 = 80 watt
n = P2/P = 80/10 = 8 pieces....infact :
4 amps require 4 parallels : 10//10//10//10 = 10/4 = 2.5 ohm
5 ohms requires 2 series of 2.5 ohm , hence 4*2 = 8 resistors
Regards