The locations of the hourglasses should be changed. They are too easily obtained and too easily defended. I would suggest moving the outer ones up/down one space.
Also, this event would be pretty epic if it you used a four-way board. To create one, you take a normal board, then add two rows/columns to each side. So it's a 12x12 grid with 2x2 squares missing from each corner. Since you aren't using the queen initially, you could change it to 11x11. The advantage of doing this is that you can keep all of the hourglasses in the center of the board, which is the high-risk area. In regular four-way chess, advancing to the center isn't the wisest of choices, as your pieces are most vulnerable there. It is much easier to attack an opponent to one side while turtling on the other.
The downside to doing that is that the event would probably take longer, since there would be more communication required.