Which Karp to evolve?

I vote for karp 4 since it has the best IVs.

Whichever one is shiny

To use as is #2
To power up #4

As long as it's over 110 I wouldn't care about the CP. Personally I think going with the highest IV is a no brainer to the point I'm wondering why you even need to ask.

Honestly, unless you don't have 800+ candy like the rest of us, I would wait on all of those for something better. At least until the end of the event.

I'd put 3 and 4 in the CP calculator and figure out who will have the highest CP at the same level (likely the 89% one, but higher IV doesn't always mean higher CP).

4 if you've got plenty of carp candy and don't mind the dust expense. 2 if you want something at a usable level without further investment.

If all are same color then I choose #3. Just my choice...

unless you have a gazillion candy or are really desperate none under 90% unless it's shiny.

but if you must which has the highest attack?

If you desperately needs to evolve one, and using it as an attacker, I would pick #3. The extra 1 attack IV as compared to the extra 3 IV in stamina means higher CP since 1 attack IV factors more for CP then 3 stamina IV.

For me, I wouldn't evolve any of those since none are over 90IV. There are so many Magikarps, even after the event that it's just not worth spending dust or candy on ones below 90IV. You are bound to find a higher IV one eventually.

highest IV, unless you are swimming in magikarp candy and already have multiple Gyarados with 90%+ IV