A change of a mere 15 minutes in one of the flights, which in no way impedes your ability to fly the ticket as booked, will not allow you to make changes for free to the ticket.

However, I should point out that you appear to have totally overlooked the issue of fare difference here, and seem to assume that the amount of €4,300 quoted is predominantly down to the fact that one of the sectors was booked in a non-refundable, non-changeable fare bucket. You can always "buy up" to a more flexible fare bucket to make changes, and that has been included in the amount quoted. Perhaps around €200-€300 of the quoted amount would be apportioned to this change to your ticket (change fee). However, the size of the quoted fee means that fares have (greatly) increased since you booked. Even if you had initially booked a fully flexible ticket, you would still now be charged an amount of roughly €4,000 because that is how much the fares have increased since you booked.

The advice to hold off and wait to see if a substantial change happens to your ticket was good advice. I'm not sure, however, that it would automatically allow you to move all dates - for both the inbound and outbound sectors - in the manner that you expect. It would, however, allow you to cancel the ticket without penalty, and get all moneys paid returned to you. Under those circumstances, you may wish to book a new ticket with a different airline, as it is likely that you would find a new ticket on your chosen dates at a much cheaper fare on some other airline.

Good luck, and please return to let us know how this resolves!