If the mean of y1 and y2 is M1
and
that of y1, y2, y3, y4 is M2,
the mean of ay1, ay1.y3/a, y4/a is
(y1+y2)/2 = M1
(y1+y2+y3+y4)/4 = M2
so, M2 = M1/2 + (y3+y4)/4
ay1+ay2 = 2a(y1+y2)/2 = 2aM1
y3/a + y4/a = 4/a (y3+y4)/4 = 4/a (M2-M1/2)
so, ay1+ay2 + y3/a + y4/a = 2aM1 + 4/a(M2 - M1/2)
= 2M1(a+1/a) + 4M2/a
better check with some actual numbers.