Random permutation of numbers in MATLAB with weights

How to randomize the numbers in a vector a, with weights assigned in such a way that I can control what numbers 'follow' other numbers?

Let's say

a = [ 1 2 3 4]

I would like to obtain something like this:

    1 2 1 3 4 2 1 4 3 4 1 3 4 1 ....

My aim is to create a longer vector (including these 1 to 4 numbers, permuted), while adding weights:

• 
  

  1 -> 2, 2 -> 1 (a 2 following a 1 in the vector and vice versa) in 1/3 of cases

  
  


• 
  

  3 -> 4, 4 -> 1 (a 3 following a 4 in the vector and vice versa) in 1/3 of cases

  
  


• 
  

  All other possible transitions, 1/3 of cases

  
  

I managed to obtain a longer vector with these numbers permuted, but I do not know how to apply these weights/rules.

Thank you very much.