Hi Denis,

Haven't done the rental thing but sure you could pick up something big enough to put boats in and transport you.  4 in a car even with a roof rack might be a squeeze so possibly a minibus or small van would be better.

With regards renting boats for a week this is possible.  2 of the main paddling shops are in central Scotland, namely Stirling Canoes in Stirling and Brookbank in Perth (both have websites).  Not sure how cheap it will be for a week but if you are all experienced (able to look after the kit) they might do you a deal.  The other option would be to get in touch with some of the many local kayak clubs in Scotland.  Perhaps pick one near where you want to paddle and they might be able to help and loan you some kit.  There is a list of clubs on the Scottish Canoe Association website.

If you are really after Grade IV - V then somewhere up the west coast, say Glencoe or Fort William would be a great base.  Things have been a little dry lately due to snow and cold temps but we must be due for some rain and snowmelt soon and it doesn't come much wetter in Scotland than Fort William.  If you've only got a week then driving all over Scotland probably isn't the best idea.  I'd keep an eye on the river levels information on this site along with the forecast then pick a destination.  Basically you want rain, and lots of it!  There are classic runs all over Scotland if you get the weather.  There is a Scottish Whitewater Guidebook which tells you all you need to know about all the rivers and there are also a selection of river guides written up on UKRGB.

Hope you have a great trip.