The level 4 windshirt or similar garment is very useful for any climate. In cold weather, use under bdu or level 5 for added protection. In warm weather, use for lite precip, wind , or insects.
I think levels 1 to 4 and 6 are the most useful layers for most conditions. Level 5 (softshells), although described as the "key to the system", really are meant for high exertion activities in cold, snowy, damp conditions. In this environ they excel and replace the BDU/ACU. Above 40F, the BDU/ACU is fine for a "level 5". The level 7 is meant for very cold to extreme arctic/alpine conditions only, although a 7a vest is a nice 3 season piece.