I cycled the four corners earlier this year. You can read about it on my blog at http://www.khain.net
There was some discussion on this forum as to what the 4 corners actually are though. I did the most southerly, northerly, easterly etc. points on the mainland (some of the true corners will be on islands), but others seemed to think different points were 'corners' e.g. Cape Wrath. What points are you heading for?
If you do the same points as I did be aware that there is nothing to mark the most easterly corner at Keith Inch (or the south-east point if you do that instead) and it is quite hard to find and a bit dismal too. Also, there are no munros near any of the corners.
The toughest thing I found was the headwinds. Doing 4 corners you will have to tackle them at some point but hopefully they won't be as bad as in March.
Have a good time