Mathematically speaking:
Creatures with CRs of X,Y, and Z
(sqrt(2))^X
+(sqrt(2))^Y
+(sqrt(2))^Z
-----------------
(sqrt(2))^N
Where N is the approximate ECL. This works for any number of creatures. For 3,4,4,6:
2*sqrt(2)
+4 [which is sqrt(2)^4)
+4
+8
---------
about 19, which is closest to 16 (from the powers of sqrt(2)), which is sqrt(2)^8, making the total ECL slightly higher than 8.