D    B

A    C


cost=sum of the length of routing between the modules

costABCD
costACBD
...
4!=24 possible combinations
find the minimum cost


Module D is made up of 6 sub-modules

4 6 3 2 5 1

Calculate the cost (routing) for each possible combination.
6!=720 combinations
find the minimum cost


Sub-module 1 is made up of 8 sub-sub-modules
1 2 3 4 5 6 7 8
Calculate the cost (routing) for each possible combination
8!=40,320 possible combinations
find the minimum cost


Etc...

Is this acceptable?
Test the speed.
Test the area.
Test the power.