I'm not familiar with NST grounding in Sweden, but I don't think you can hook up two 4 KV NSTs serially for 8 KV due to center tap grounding. Before you toss your MMCs out you should take a look at your NST connections.
Stork
Stork,
Many of the lower voltage NST are single ended rather than center tapped. Perhaps he used single ended units which should work out fine if he grounds the center point of the two NSTs.