If you have to go four days anyway and you're one of the people like me who doesnt see the point in going in to uni for 2 hours and leaving, then I'd try have a nice even spread across the 4 days.
However if you have a part time job and want to have better availability for work then you may want to load up on 2-3 of the days and just have the auto allocated units on 1-2 of the days so you can go to work/do other things that don't involve University.
From the perspective of what's probably best for your education I'd say try have a nice even spread so you can take in whats said in lectures etc. Also, depending on how good you are at studying (edit: this is meant to mean, how good you are at studying without getting distracted at home by other things) you might want to consider having some 1 hour breaks each day to study in the library. (I've heard many Uni students say that that's where they get most of their work done. Depends on the individual though obviously.)
Also, I've been advised to try put allocate in some breaks for lunch etc.