There must be a extremely light .308 bullet out there, that can handle the velocity. Just remember that 4,000fps loads tend to lose velocity extremely quickly, due to the combination of the light low BC bullets and the increased wind resistance from such a high speed. Wind resistance tends to grow exponentially with speed.
A 4000fps load with a light round nosed bullet could lose close to 1000fps by the time it hits the 100 yard mark. If you're going to be shooting that far, it may make more sense to load something like the 110 grain V-max to max pressure, as you'd end up with a similar velocity (If not more) at 100 yards, with more energy, and projectile mass, as well as less wind drift. Time of flight will be about the same, and only about .2'' more drop at 100 yards. It's still laser flat, and should be just as 'explosive.'