Complex sentence: If the improved M4 turns out to be the winner and the new carbine program is scrapped, then the IBCTs will likely be fitted with the improved M4s, and the existing M4s would again be given to support troops to replace their M16s.
Split into simpler sentences: If the improved M4 turns out to be the winner, and the new carbine program is scrapped. Then the ICBTs will likely be fitted with the improved M4s, and the existing M4s would again be given to support troops to replace their M16s.