Perform each function based on order set by user

Well, I can only allow the user to perform in such a way:

Function 1 > Function 2 > Function 3 > Function 4

What I wish to create is I can freely change the order on which function to perform (Maybe by allow user to set the sequence number on the function)??

For example:

Function 3 > Function 1 > Function 4 > Function 2

Any idea or hints on doing so??