the same amount is shown on my credit card statememt 4 times, as itunes luxembourg - but we have no record on the account

We are fellow users here on these forums, we won't know what they're for for.

You've viewed the purchase history on your account via your computer's iTunes : log into your account on your computer's iTunes via the Store > View Account menu option, you should then see a Purchase History section with a 'see all' link to the right of it. Click on that and you should then see a list of your purchases.

And they aren't temporary store holding charges : http://support.apple.com/kb/HT3702 ?

If nothing shows on your account, and they aren't holding charges, then you could try contacting iTunes Support and see if they can help : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

But if nothing does show on your account then you may also want to contact your card issuer.

I think that may be how iTunes charges may appear for some customers in the EU.