What speed and gear do you get under 4 w/o LB and w/o slowing down so much that you have to start applying more throttle?LB is not the holy grail. My Ultragauge tells me when my car does LB, and ist does it quite often although it's not perfect (missing belly panel, steel rims, all season tyres).
So what I have learned: even without LB is is a fuel saving car. It can drive below 4 l/100 km without doing LB. But I know, that's only my driving profile.