no real reason at all. mainly I've never built a set of 4x wheels. they'll be going on a lugged track frame and I thought 4x would look cool, a little old school, and I'm not too concerned with weight.
Make sure the 4x pattern will work on your hubs. There is a better chance that spokes will cover the head of the spoke adjacent to it. This problem makes changing broken spokes very difficult. The problem is more likely on hubs with a large flange diameter.