I have another question: Why is the bus tie necessary? Could you simply put an extra transformer in the system with a breaker going to each bus in place of the tie? This way you could switch on the 5th transformer in the event you need extra power on one of the buses and not worry about ever connecting them. Granted, not the cheapest solution, but perhaps less maintenance. You could also spec this transformer to have a higher impedance and possibly a lower KVA rating which would lower your fault currents significantly.
I also like davidbeach's suggestion.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.