Been to one 4-day convention: AnimeFest in Dallas (didn't make it this year, unfortunately).
It's a bit disorienting. Kind of like having 2 Saturdays. You get done with the usual Saturday festivities and realize you have another full day and night, instead of the Sunday pack-up. And you have to make adjustments for it, in food being brought, in other supplies, adjusting hotel costs, etc. We'd have to rent a whole other truck for ramune

Pluses:
- spreading events out. Don't have to worry about a concert, cosplay and dance all on Saturday.
- More attendees without breaking the cap. Since the cap is calculated daily, you could have more one-day attendees.
Minuses:
- More time off needed for staff, attendees, vendors, etc. Especially if it's not on a holiday weekend.
- May not make difference to vendors. My experience at AnimeFest says that, outside of snacks, people just spread out their purchases.
- More content to fill. If there is any problems filling the panel schedule, it could be made worse by adding an extra full day.
Right now, the closest holiday weekend is Labor Day. Moving NDK to it would put it in competition with 7 other conventions, including AnimeFest and Dragon*Con. This may make it harder to get guests. And I doubt they will make 9/11 a national holiday, or at least one that makes a 4-day weekend.