I had the 2nd gear to 3rd gear grind problem in my Ford explorer. I was told by the dealer that it was the synchros breaking in. Eventually it went away. But the 4th to 3rd downshift, I would take it in to the dealar to check it out. That could be a shift fork issue. Being that I had a Ford, problems were expected. But with a Nissan being such high quality, I would be a bit concerned. Hope this helps. Good Luck!!