Would the easier thing to just have the three others back-to-back, then have a 'jam' at the end of the last one's set (with members of the three bands) doing, maybe, one of each others' songs plus a couple of 'tallica tracks? Maybe get Jason Newstead out on stage with them too?
The easiest thing? Surely having Megadeth, Slayer and Anthrax back to back is completely out of BOA's league too, with the former 2 being festival headliners twice already, and Anthrax being very high profile special guests, and almost headliner level. Metallica are Metallica, they'll not play BOA, but at the same time they haven't played much larger festivals like Wacken, Hellfest and Graspop, they're absence is hardly an issue, and we don't need to make up for it by a loada covers.
Back to the Teutonic 4, how would you book the undercard / 2nd stage to compensate for all that thrash (and the fact that the bands will mostly be playing bigger slots than they usually do)
The Teutonic 4 is within the grasp of the festival finance wise, but it wouldn't make sense to have it all in a row at the top of the bill, Wacken yes, BOA no. The only way would be spreading it over the weekend, which would defeat the point a bit. The only way to really draw attention to it would to be having Destruction, Sodom and Tankard each headline a night in the Sophie tent and have Kreator headline a mainstage night.