If you have never been on big water then turning up mid May or June can be a bit of an experience. Most UK boaters turn up in July or August when it is nice, warm & sunny and most rivers have a good level.
Most of the classic runs in Tirol are grade 4 big volume and continuous. So when it says 9 km of grade 4 there will not be any flat water in there rather a few bits that are just grade 2 or 3 (a bit different from a lot of UK runs). On some sections of the Inn the banks are a long way away so having a good roll is more important than in the UK for the same river difficulty, as well as having a bit bigger boat.
The white water Europe - North alps guide book is a good buy for lots of imformation.
Neil Newton Taylor
www.swiftwaterrescue.at