Yes, WDG4 is WDG4D.
Either it is a typo, or WDG5 was just an experimental version of the WDG6. A 6000HP EMD was proposed more than a decade ago, and IR opted for a complete in-house development using the 16 cylinder engine from GM-EMD. The WDG5 corresponds to everything mentioned in the initial proposals (only the engine power is off) and by design at least, should be the best freight loco in the country . May be they figured out a way to extract more from the power-pack and are upgrading it to G6. ( Hope it wasn't a typo !!)