I think this year (and especially if he gets YE#1) cements his place in history. Last year he ended #2, made a final, but still only had the 2 slams and the on OG, but made nice DC win. This year really did it for him. Extra slam, an 2nd OG singles medal, which may not count as a slam but is so unique it actually sets him apart from ALL OTHER tennis players, finals on all slams, the consistency that he's had, AND showing how dominant he can be after Djokovic started faltering are all pretty big things.
He has the YE#1 like the rest of the big 4. They all did it. They've dominated together since 2008 basically, and it wasn't until 2016 that two of them were to declined to do anything. They all have slams, bunch of masters, and now that Murray has the 2 Olympic singles Gold, they all have their own unique record. Murray is still in it with a fighting chance to complete the whole palmares with 3 masters, 2 slams and WTF, all events at which he's got chances. Not saying he'll do it, but it coud happen.
For me, the Big 4 was more about consistency and top level than about distributing prizes evenly. They'd all just reach semi's of 80% of big tournaments they played, and for each and every one of them was the heavy favourite against any guy from the 5-10 in the world. They've been together as top 4 for the longest time and they've all had their epics together (maybe except for Murray-Nadal). In later years, Wawrinka came in as a wildcard basically.
The Big 4 doesn't mean they were equally big. It means they were the biggest 4, and the 5th best player was far, far away during that period, and I love, that in the center of that period, they had a year in which they all won a slam with 3 of out of the 4 winning 'their own' best slam. (with Murray, instead of winning Wimbly, winning USO and Olympic gold @ Wimbledon). That's just beautiful symmetry to me.