When I've tried Millers Sport 4 in the diesels I've owned over the years I've found it made no difference whatsoever and a complete waste of money.
Unlike BP Ultimate. I put £60 in yesterday and to be honest it actually feels livelier with fresh diesel just after filling up, straight out of the garage. I've heard that diesel sitting in your tank for a while it loses its cetane level (and likewise octane for petrol). Its only a slight difference but noticable therefore these days I no longer fill up with a complete full tank and have it sat in the tank for 2 weeks.