Can I do a cavallery charge (assault over two hexes) if the spotting range is only one hex at night?
I would say no but I found no rule which prohibits this.
The first sentence of 15.31, Cavalry Charge, states: "Cavalry units activated by a Cavalry Leader may conduct assault movement (12.1) from two hexes away against a unit it can spot".
Thus cavalry units cannot charge at units two hexes away if the spotting range is 1.
Fourth edition rules can be found on this site at the URL below. They can be accessed from the main page by selecting "Annotated Rules" from the main page and then selecting "4th edition".
A number of things were clarified in 4th edition. I still want to produce illustrated examples of play at some point but time is severily lacking these days with development efforts and looking for another job taking up most of my free time.