4X yes, never heard of anybody running 5X, however it would most likely be possible, just need longer spokes and some way of lacing them. Although since 3X is adequate for most people, and 4X for anyone else, it's hardly necessary really, weight:strength addition ration not really much (as in for the weight you're adding you aren't gaining much more strength) Just run a nice old school roadie solid (disc) wheel!