Well in comments Rory McCrossan mentioned . Yes the combined cells of table should be 100%
Well in your case there are 4 columns right then its pretty straight farword .
"aoColumns": [
{ sWidth: '25%' },
{ sWidth: '25%' },
{ sWidth: '25%' },
{ sWidth: '25%' } ]
And if you want to do it in a different way you can set bAutoWidth:True .
Regards