If $300 is no big deal to you and you'd be happy with the library even if there wasn't another game released for it other than Zelda .... sure go for it. There's fun to be had with it just like any other Nintendo system and there's a lot of Wii and Virtual Console games to play as well that I'm sure will go well with your 4-year-old. Honestly I'm not sure he'll be all that great at Splatoon though, I'd say that's more of a game for somewhat older kids (and beyond).
But know what you're getting into. There is a high probability that Nintendo has basically stopped major support of this system and is moving on to the NX. If third parties have dev kits that means Nintendo's in-house developers have likely had them for a long time. Given your son is 4-year-old, I think this is more than enough game system for him probably until he's 7/8 then he will start asking for a new system, but I guess that comes with the territory of having kids.
Since it's for a 4-year-old, like I said I'd probably lean towards "yes", but I would try hard to see if you find a discounted Wii U somewhere or refurbished one from Nintendo for like $199.99.
https://store.nintendo.com/ng3/browse/productDetailColorSizePicker.jsp?productId=prod150200
$199.99 w/Nintendo Land and 32GB is not bad and it's from Nintendo so I believe it has a full warranty (most of the Nintendo refurb units from what I understand are basically brand new, they're like deadstock inventory).
I'd get that and then maybe grab Mario Kart 8. I think Splatoon might be too intense for a 4-year-old. Nintendo Land actually is quite a bit of fun with kids.
If you're still kinda iffy ... I might think a Wii might be a decent stop-gap choice. You could probably get one for $40-$50 and Mario Kart Wii and New Super Mario Bros. Wii and Punch-Out! are dirt cheap (things a 4-year-old could probably play easily).
