Unlocking potential

I have one 4* Nino and three 3*s. i want to make one 5*. And I know when I merge ones with the same star level their stats go up, right? My plan was to level up the three 3* to 4*, and then merge them to get one 4* which would be +3. And then make that one a 5*. That woul cost me 26k feathers. But then i realized I could first merge the three at 3* level, and then promote this one to a 4*, then merge the two 4*, and then the remaining one to a 5*. That would cost 22k feathers. My question is does it matter if I merge them at 3* or 4*, is there a difference in the stats? I know that I will get more sp if they have more stars, but sp isn't a problem to grind. Also, should i spent my feathers on Nino? I'm in need for green and blue magic heroes. The only blue I have is robin M, and green nino and cecilia.