To get more depth-of-field you have to increase the aperture from F/4. Since your subject does not have much depth, F/8 might be enough. Otherwise, try F/11.
Since you are using flashes, their illumination will drop off proportionally to your aperture. So going from F/4 to F/8, you loose 2 stops of light. There are 3 ways to compensate:
- Boost ISO by 2 stops, so to ISO 800 in that case. In your D300s, the quality difference should be minimal. This is the easiest adjustment to make.
- Reduce the distance of the flashes by half. Light falls off with the square of the distance. So at half the distance, you will have 4X the flash power.
- Boost the flash power by 2 stops.