shortcode only 4 digits

[kikipluche]

Lion suggested to change the shortcode from 5 digits to 4 digits (possibility of 10000 people doing a transaction at the same time), the 4 digits can be shown split2 (12 34) so people can easily remember the shortcode. when you are the sender entering the shortcode is in 2 groups of 2 digits too (12space34).
@xardass @faffydee how do you feel about this change?