I've no idea how mobile companies operate in Germany.
What I'd suggest is if you want a comparable phone but without the premium which Apple products incur then you may want to look at an Android phone such as the HTC Sensation or Samsung Galaxy S2.
See if they will let you upgrade to that at a cheaper tariff? Also, bear in mind that the Iphone 4S/5/whatever is due out late Sept, early Oct so it might be worth you hanging on for a little while longer.
Even if you decide not to upgrade to the new model, chances are the iphone 4 will become cheaper and other Android competitors may offer discounts to woo customers. Either way you should win compared to upgrading now.