Prior to PD4ML v4 our licensing model required to purchase an additional single license to cover development environment. For v4 we removed the requirement, however there are opinions here to return it back. We are collecting statistics now to make the final decision later.
If you order a single license right now, it will also cover your DEV needs – according to the current licensing terms. PD4ML Volume licenses (now they are called PD4ML Dev Site) always covered and are going to cover DEV environment.