The 4" coil will certainly work, but, in wide open, low trash ares, you are probably better with a larger coil. The smaller coils work great in heavily trashed areas where you need better separation of targets. In large, open, areas with less trash, a larger coil will give better coverage and more depth.
If both your coils are standard Bounty Hunter coils, they are most likely concentric as opposed to DD. It is recommended that you do a 50% overlap per sweep with concentric coils, for proper coverage. With a 4" coil that would give you an effective coverage of 2" per sweep. Hope this helps.