Hi Jerry,
thanks for your advise. I might already have a suspect:
http://www.bilder-hochladen.net/i/m0et-4-7a42.jpg

I've no gas torch at hand right now, but will check as soon as possible. But since the intake ports are paired, wouldn't a leak which affects cyl. #3 also affect cyl. #4?

Further question: Will it do any harm to the engine if I drive the truck this way?