Is J4 support a thing?

I recently had a game with j4 adc with blitz support. If blitz lands a hook, the full combo is instakill (no counter). So if you take him with a bursty adc, it could be scary.

Can it be a thing? Sure. I've both played J4 support, and had a J4 support when I was ADC and it has gone well. He brings lots of things you'd want to bring as a support - good base damage with his passive, AS buff for his ADC, armor shred for his ADC, a cc knockup and a slow, a shield to be tankier on low gold, and a CC ult with decent base damage.

Is it legit/meta? Not really. It can work but it depends on snowballing a bit too much. Leona does pretty much everything better as a support. I.e. if you can afford a damage item then J4 is better than Leona (why you don't often see Leona in the jungle or top lane, but J4 is there) but if you can't afford to invest in damage then there's not much reason to pick J4 over Leona. Maaybe there exist team comps for it (enemy has Gangplank top, Annie mid, Varus ADC, Morg Support [i.e. a team with no dashes] and you have like Anivia mid and Jinx ADC [i.e. lots of AOE damage]) where J4's ult gets maximum value, but in general there are support champions who can do the same stuff as J4 better on that income level. J4 will have a much better levels 1-2 than they do (and possibly even 4-6 or in general until your first back) but once you start getting minimal items, J4 needs some damage to scale whereas other tanky supports will do much better without the gold income.