list1 = {a1,b1,c1,d1}
list2 = {a2,b2,c2,d2}

sumList = {a1+a2,a1+b2,a1+c2,a1+d2,b1+a2,b1+b2,...,d1+c2,d1+d2}

not elements such as a1+a2+b2...