PSV (and Dutch players in general, seeing as though Ajax have also won it) certainly seems to be a a helpful link here. But a lot of the possibilities that I've thought of I'm sure won it with Ajax in the mid 90s. Robben and Van Der Vaart are two that obviously weren't in that Ajax team, but neither have played for 4 winning clubs.
There must have been some big name Dutch players that have played for Celtic. Why can't I think of any.
I've got a feeling that the answer to number 2 is going to be someone before my time. Hargreaves is only English player that I can think of from my era to have played in Germany (and I'm guessing it is an English player).
I decided to ditch the "which English players can I think of that have played in Germany" strategy in favour of "which German players can I think of that have played in England". I got all excited at one point when Didi Hamman came to mind, considering his spells at Liverpool and Newcastle instantly stood out.
I have to apologise to you Brendy, i've just checked Van Nistelrooy and Zenden and they are both correct. The question I asked must have needed to be updated as there are actually 6 players not 4. So that just leaves 2 to find.
Oh damm it! I thought I was done and dusted, got the 4. I'll have a think who the other 2 are in a bit.