SNES - Turned 4 in late 1994 (Japan), mid 1995 (NA), early-mid 1996 (PAL) - Notable games: Original DKC trilogy, SMW2: Yoshi's Island, Killer Instinct, Dragon Quest VI, Chrono Trigger, Super Mario RPG + a lot of high quality 3rd party titles. And with hardware sales still relatively steady, enough to breakaway from the Genesis and win the 16-bit Console Wars and had a great sendoff before the N64 arrived. Yep, Super Nintendo is still the king. (For now)
N64 - Turned 4 years old in mid-late 2000 (Japan and NA), early-mid 2001 (Europe and Australia) - Notable games: Majora's Mask, Conker's Bad Fur Day, Banjo-Tooie, Pokemon Stadium 2, Paper Mario, Mario Tennis, and Mario Party 3; Pretty solid list with a strong first few titles, including what I consider the best 3D Zelda game pre-BOTW. But that was about it, aside from the few 3rd party games it received that were far better on the PS1 and Dreamcast anyways. And sales had dropped drastically at that point with just 1 year before the GameCube came out, so not what I'd consider a good life post-4th year.
GCN - Turned 4 in late 2005 (mid 2006 in Europe and Australia) - Notable games: Twilight Princess, Pokemon XD: Gale of Darkness, and Mario Party 7; ...Damn, that's pretty pathetic. Nothing against those three games. I love Twilight Princess to pieces, but it was also the Wii's launch title so that weakens the impact for the GameCube. Never played Gale of Darkness or Mario Party 7, so I can't comment on those. But you could tell Nintendo gave up on the GameCube at this point and they were chomping at the bit to move on to the Wii.
Wii - Turned 4 in late 2010 - Notable games: Skyward Sword, DKC Returns, Return to Dreamland, Xenoblade Chronicles, The Last Story, Sonic Colors, Mario & Sonic London Olympics, Mario Party 9, and a crap ton of 3rd party titles (even if at least half of them were shovelware.) Another solid list and I'd say even stronger than the N64. I guess the Wii had a better last few years than I originally thought it did. Now that I think about it, maybe they should have rode the Wii for another year so that they'd have time to make sure the Wii U's launch lineup was strong & loaded, and so they could iron out the kinks and fix easy mistakes they made early on (like the name and marketing.) But then again, if they did that, we'd probably have no Switch right now, or at least not until just very recently. So perhaps it worked out better this way.
Wii U - Turned 4 in late 2016 - Do I even have to say it?
TL:DR - It seems like ever since the Super Nintendo, the last one to still have a strong and healthy life post-4th birthday, every other Nintendo console noticeably dropped off in terms of sales and software releases. But the Switch looks poised to break that jinx and really flourish in the 2nd half of its life. (Provided Nintendo doesn't screw it up, which they unfortunately have a habit of doing.)