Theres nothing to clarify really. You need to drain and refil it 3 times, with at least 5 km worth of driving in between changes to make sure the new fluid mixes with the old. You'll notice say for example, 3L comes out of the pan but the manual says the tranny holds 8L. Its same with almost all trannies out there, old or new. So.. that 4L you bought is definitely not enough unfortunately. You need another 2 bottles to do it correctly.
Btw Matic D is very close to Dexron III (Transmax Z is rated Dexron III). I have had no problems using Dexron III products in Nissans and some factory manuals also suggest Dexron III if Matic D is not available. Also my ignorance suggests that Matic D is not synthetic (as if Nissan would go synthetic for anything from factory!) Also pulling the tranny lines won't do the convertor. It'll only drain whats in the pan, even less so because there will be some left underneath the oil pickup.
Cheers.