Un-average temperatures!
Python, 63 bytes
def f(a):x=(a+183.205)*5/7;m=x*9/5;return x-273.15,m-459.67,x,m

a is the average, returns a tuple of the results as (celsius, fahrenheit, kelvin, rankine)
Math involved:
kelvin = x
celsius = x - 273.15
fahrenheit = x * 9/5 - 459.67
rankine = x * 9/5

a = (x + x - 273.15 + x * 9/5 - 459.67 + x * 9/5) / 4 = x * 7/5 - 183.205
x = (a + 183.205) * 5/7