#(a+b)/(a+b+4)* (4a^2+4b^2)/(a+b−4)*(a^2+2ab+b^2−16)/(16a^4−16b^4)#
#=(a+b)/(a+b+4)* [4(a^2+b^2)]/(a+b−4)*[(a+b)^2−16]/[16(a^4−b^4)]#
#=(a+b)/(a+b+4)* [4(a^2+b^2)]/(a+b−4)*[(a+b−4)(a+b+4)]/[16(a^2−b^2)(a^2+b^2)]#
#=(a+b)/(a+b+4)* [4(a^2+b^2)]/(a+b−4)*[(a+b−4)(a+b+4)]/[16(a−b)(a+b)(a^2+b^2)]#
#=cancel(a+b)/(a+b+4)* [4(a^2+b^2)]/(a+b−4)*[(a+b−4)(a+b+4)]/[16(a−b)(cancel(a+b))(a^2+b^2)]#
#=1/cancel(a+b+4)* [4(a^2+b^2)]/(a+b−4)*[(a+b−4)(cancel(a+b+4))]/[16(a−b)(a^2+b^2)]#
#=1/1* [4(cancel(a^2+b^2))]/(a+b−4)*(a+b−4)/[16(a−b)(cancel(a^2+b^2))]#
#=4/(cancel(a+b−4))*(cancel(a+b−4))/[16(a−b)]#
#=4/1*1/[16(a-b)]#
#=1/[4(a-b)]#