4*+ or 5*

A 4*+5 has the same base stats as a 5* non-merged, just without the weapon and a skill maxed. For Nino, It would probably be better to get a +9 4*, as her stats will be higher than a basic 5*. Nino doesn't need to be 5* to destroy other units.
I have a Nino 4*+6 and I haven't even considered merging her. I would rather keep my feathers to promote a unit that needs it.

I would promote her to 5* and slowly build her. A merged 4* will max out quick and at that point you might be disapointed she can't go farther. You'll also need to 5* at least one to get gronnblade+. A 5* unmerged is very potent already, and she'll have much more potential to grow. Good luck whatever you choose :)

I would promote her to 5* and slowly build her. A merged 4* will max out quick and at that point you might be disapointed she can't go farther. You'll also need to 5* at least one to get gronnblade+. A 5* unmerged is very potent already, and she'll have much more potential to grow. Good luck whatever you choose :)

The result is ridiculously obvious. Not only does she end up getting more milage in much, much, MUCH shorter time as a 4*+9, but it's also millions of times cheaper and outright completely unnecessary to go the other route unless you just want to brag about it online (in which case, you'd already be losing). And not only that, but the extreme Blade tome bonuses are to heavily make up for the deficit vs. the *5 version.

People ought to be more realistic in their expectations. Unless you're willing to either wait something like months to even a year to spend on one single unit, you could be composing a whole team based on actual resources you already do have.

As to IVs, I'm not sure what you mean what "only one has good IVs", but I take it you mean +Spd and not -Atk. If you can, merge a final one into that or +Atk (assuming she's already doubling everything as it is).

If you merge her as a 5* slowly, her stats will be extremely high. It obviously takes time and dedication so unless you really value Nino don't do it. I probably wouldn't because it's overkill. I plan to get 5* merges on my Tharja only because she's my favorite.

I have a +5 4* Lukas (I was trying to snipe Reinhardt during Vantage banner) and he has great stats. I gave him Brave Lance (not even plus) and run Atk+3 and he can consistently wall off and nuke reds. In any case, use the best IV as the base for the merge.

always go for 5. In the long run, merging 4s will only get you so far.