First off ill say ive never ridden, built or even seen a 4-cross wheel in the flesh. Im an ameuter wheel builder and not the most skilled however ill happilly throw myself off cliffs on my mtb wheels or hit 50+mph down a col on my road wheels without worry as im confident the components used are the best you can buy (or at least ones i can afford ;0). Its really not that hard a skill, and using a good wheelbuilding book as a guide ANYONE can build a strong wheel. I reccomend Roger Mussons guide (
www.wheelpro.co.uk).
The benefit of running 4-cross over 3-cross (as i understand it) is the lower tension in the spokes, however the use of high quality double butted spokes should allow enough 'stretch' for this to not really make an impact.
Obviously this lower tension is also a benefit in a 36 spoke wheel but the other advantages of a 36 spoke wheel are the ability to handle larger side loads due to the rim being more supported and the ability to stay true when a spoke breaks (an issue if you run rim brakes). However modern rims tend to be MUCH stronger and stiffer than when wheels were all being built 36spoke, and these issues are not as much of a problem as they used to be. I have built, ridden and thoroughly beaten many 32*3cross wheels in my time and if you use appropriate good quality DB spokes and a decent rim then you should have no issues at all.
Hope that helps :0)
Rich.